I highly doubt he is mentally handicapped if he is already getting comments on how bright he is in a school setting!
I don't know if he's autistic or not; obviously. But if he is, well, he's in mainstream school with no problems and is thriving. So he's clearly doing really well. So you have nothing to worry about. If he is autistic, so what? He's still your son and you won't love him any less and he's still the same person he's always been.
I doubt he is because it sounds like he handled the transition into the new situation very easily and without fuss, which is something that can be hard for kids with autism.