Flying is completely asymmetrical.

Because a flying creature can move in 3D, they have literally infinitely more options for movement and positioning than do grounded creatures. There is absolutely no response to it for most creatures.

Ranged attackers tend to do OK, obviously, and of course anyone else with flight is fine, but melee, grounded creatures are usually out of luck. A back-up ranged weapon is rarely sufficient to actually threaten flying creatures, as generally one will not have the feats or class features to support it, and there isn’t really any other way to deal with the situation.

This is just a fact of the game system. Once flight becomes available, it becomes imperative that todos gets flight ASAP. It’s often cited as a major reason why the weaker classes estão weaker.

As the DM, your only real options are: design every encounter with ways of responding to flight, or make sure all meaningful combats take place in an enclosed space that prevents flying. Both of these are extremely limiting, unfortunately, but they are the only real ways to deal with it.

Or, you start houseruling things. My personal choice is my abstract flight variant (technically it’s for 3.5, which means there are no references to the flight skill, but adapting for Pathfinder is fairly simple) – it makes flight fairly unrealistic, and accepts occasional breaking of verisimilitude/immersion, but it’s massively simpler to run and makes flight not quite so one-sided.

Fighting flyers can be frustrating to flightless creatures. Fortunately, there are some tricks a monster can pull against a flyer, and further means for you as a DM to limit or negate the flyer's advantages.

Before diving in, there's something worth noting: If your player is genuinely excited about playing a flyer (and not just about a cheesy "unbeatable" maneuver), it is probably better to find ways to challenge him without "clipping his wings". Let some enemies wipe the smirk off his face - but don't make your game a series of anti-aircraft Strix seeking missiles...

Tricks for Poor Grounded Monsters

Depending on the monster's cleverness, form and arsenal, it can:
(Of course, you as the DM can arrange for these options to be more or less valid by planning ahead)

1. Use ranged weapons/effects - this isn't limited to foes with bows - an angry ogre can easily fling a rock at a pesky flyer, and more exotic creatures may spit poison, shoot thorns, or use a breath weapon - there are also countless ranged spells and spell-like abilities that achieve stuff like that...
2. Catch him - Reach him and start a grapple: jump, climb a tree, shoot a sticky tongue, throw a net - whatever it takes to make contact and hold him. If it's a big heavy monster, the flyer's now grounded. If it's a small one, it is now piggybacking, beating the flyer and annoying him just as much as he did a few rounds ago...
3. Ground him - aside from grappling, a monster may be able to paralyze, atordoar or atordoar the flyer - denying his ability to fly even for a round (than you can take care of him...). With DM's discretion, throwing ropes, bolas or tangle-foot bags may also be means of forcing him to the ground.
4. Remove his weapon - ready an action to desarmar or separador his reach weapon (the weapon is touching the monster, so it stands to reason the monster can affect it even if it can't reach the wielder...). Exotic foes may Ferrugem lo, Cola it or cast Heat Metal, Graxa etc.
5. Use o meio ambiente - it's easy and fun for a flyer to stab his foe to death in a flat desert, but it's a whole different story if there are caves, thick trees, buildings etc. in his way. Run away to a place where the flyer can't fly safely or can't attack from above at all. Some foes may even be able to create a hazardous environment - using spells such as Web, a horizontally placed -Wall, Move Earth and so on; Or escape by burrowing, Melding into Stone or diving into water.
6. Give him something "better" to do than attack - injure his friends to the point he is forced to land and treat them or they'll die, summon a swarm to distract him, set fire to buildings of the village he is protecting, scare away the group's horses, confundir/dominar/hipnotizar him etc.
   Observação: As a DM you can even make this interesting and rewarding for the flyer - for example having an ally hanging for dear life from a cliff (maybe he was pushed, maybe he is tied in a hanging cage, maybe he just fell...) - give the flyer something unique and heroic to do instead of just stabbing away...
7. Get some more able friends - just because it can't fly, doesn't mean the monster doesn't have a pet / ally that can. It can have a trained flying attack creature or mount. It may be able to control plants or convocar creatures which can either fly, have better height/reach or can better apply any of the other strategies suggested here.
8. Desaparecer - using spells, stealth or environmental conditions (darkness, fog etc.) become hidden. If he can't find the monster, he can't attack it. This is useful for escaping or as a way to improve the odds of pulling off one of the other strategies.
9. Stand on a fast moving object - tricky without the DM specifically planning this, but, if the fight is aboard a fast moving carriage/ship/blimp/super-massive-dragon-turtle, than the flyer can't just hover and stab - in-fact, if he goes airborne, he'll be left behind and won't be able to catch-up (at least not while fighting). This applies for similar conditions such as extremely strong wind, a fight while sliding down an icy slope (on either proper sleds or logs and derbies), aboard an elevator-platform, or a falling object.

Can't think of any more right now, if anybody reading this has more ideas, please feel free to add them.

p.s. I know that some of the ideas above may deviate or slightly bend the RAW, I tried to point it out where this is the case. I hope this inspires you to think outside the box and design challenging and entertaining encounters for your party, flying PC's player included...

So @KRyan is completely correct that flying is asymmetrical. Over time this becomes an absolute problem for really everyone. Your 12th-level two-handed fighter is basically useless against a dragon unless he's running around with a flying carpet or some version of the Voar soletrar.

As Pathfinder is designed there are really two important methods that melee warriors use to control the board: reach and flight. Normally these things are accessible slowly over time.

But in your case, the Strix fighter has jumped this curve. He achieved flight e reach at level 1. That's a big deal. Most 1st-level monsters simply don't have an answer for this.

Ways to work with this at early levels

• Intelligent foes with range weapons. It's perfectly acceptable for a swarm of Goblins to have a few archers. In fact, I would kind of expect any organized group of Warriors to have a few ranged experts along with the melee experts.
• Really understand the Fly rules and Armor Encumbrance. If your player is trying to hover they need to make a DC 15 Fly check. If your player is trying to move less than half of their movement, they need to make a DC 10 check.
• A flying creature that takes combat damage needs to make fly check of 10 or he drops 10 feet.
• If they fail the check badly they fall! If you are using wings and you fail a Fly check by 5 or more, you plummet to the ground, taking the appropriate falling damage.
• A level 1 Strix is likely going to have a Fly skill of 5 or 6 (Class Skill + 1 rank + 2 or 3 DEX). At that level, your Strix is going to fail the hover check half of the time and they're going to simply fall a quarter of the time.
• If they have any armor better than leather, they are going to get an armor check penalty as well. So they are definitely não flying around in Full Plate until much higher levels. Even Armor Expertise doesn't fix this until much later.
• If you take into account the dramatically reduced armor class, the flying Strix is particularly weak against ranged attackers. At level 3, his grounded buddies are running around with AC 20 (full plate), but the reach Strix probably has AC 15 or so to avoid failing his fly checks. That's going to make him a much easier target for opposing Archers or even just Wizards who can knock him down 10 feet with a couple of magic missiles and bad Fly check.

Honestly, the broken version of a Strix fighter would não be the reach version, it would be the Archer version. But don't tell your PC this :)

At least at low levels, this is not as one-sided as it appears. Hovering like that requires a DC 15 Voar check to remain in flight, and until the character has invested some skill points and gained some levels, that's not an automatic success.

At later levels the problem remains, although there are other sources of flight available by this point.