If he can speak, walk, use a computer, etc., he's not at the level of an infant.

BUT. More importantly, he's still subject to the rule of law as an adult (if he committed a crime because he didn't know something was illegal,that isn't an excuse. You could argue as his defense lawyer based on diminished capacity, but apparently he doesn't have that, right? He can be told, "don't steal" and he won't? His mind does work?)

So yes, he should be able to vote and have adult rights. If you were his caretaker, I think it would be likely that you'd want to discourage alcohol use or sexual involvement until he re-learned things, though.