so can I purchase his Metro ticket and mine with only my ID?
Não, a fotografia está anexada ao passe - e precisa corresponder ao usuário (de Paris de trem ):
How to buy Navigo Découverte pass
You can buy a Navigo Découverte pass at nearly any Metro, RER, Transilien train ticket window that normally sells tickets and passes. (I say “nearly” because a ticket window may run out of blank Navigo passes to sell you.) You’ll need a smaller than passport size photo to be used on the pass (exact Navigo Decouverte photo size is 3cm tall by 2.5cm wide), plus a pen to write your name. There’s nearly always a photo booth nearby ticket windows at major stations, costing 5€ for a set of four pictures, so be sure to get your photo taken before going to the ticket window where they’ll likely have scissors handy for trimming your freshly minted photo on the pass. If you’re hesitant in how to ask to buy the pass at a ticket window, try printing out this page and showing the ticket seller the image of the Découverte card on the top right while holding up your small headshot photo. I’m pretty sure the ticket seller will get the idea.