Medical science is improving all the time and he obviously would have got the best treatment possible. There is a chance he can play again, the brain also has its own powers and the motivation of playing competitively may even increase his recovery. But all of that is secondary, his health is paramount, more so that because he has kids. I'm never a fan of players playing while injured and he should definitely do what's best for his health.
If he can play as a full back, he may have to be a bit more conservative than before. I could also see him coming back in central defense instead of left back. I'd actually really like that, because if he transitioned to 4th choice CB that would be a wonderful troll to all the Hummels fans on the board lol.