Esta é quase certamente a série Last Legionary de Douglas Hill, que eu também dei como resposta aqui: série de romances jovens-adultos de ficção científica. Garoto treinado como guerreiro, todos os outros garotos guerreiros eliminados
Em Wikipédia :
As the book begins, Keill Randor is the leader of an off-world Strike Team, far from home. He receives a distress signal from his home planet Moros, stating that they are under attack from unknown forces. Keill's ship has suffered damage, and he is unable to immediately return to Moros. When he arrives, the entire planet is covered with radiation. He is met by Oni's ship, and she tells him to flee the planet; a deadly radiation is killing everyone who comes close to the surface.
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The Overseers also explain that, since the radiation sickness had settled into his bones, they were forced to replace his entire skeleton – using an unbreakable alloy – effectively rendering Keill's bones unbreakable. They also reveal how they captured Keill so easily – a telepathic avian life form from another galaxy known only as 'Glr'.