Why? He has in coma for at least 5 years, I'm fairly certain you don't just wake up and walk away. That he hasn't showed up anywhere has a few simple explanations;
1. He wouldn't be able to cope with the stress.
2. Depending on which part(s) are damaged in the brain he might function less.
3. There'll probably be quite a loss of muscle, making it physically impossible.
4. He'll need to adjust, a lot.
Just because you're "alert and coherent" does not mean you'll be capable for the outside world.