I completely understand, as my son is also academically above his class, but has been a bit behind emotionally - complete with extreme sensitivity at times. My advice - don't stress about labels (autism, ADD, etc.). It seems that everyone is so quick to put labels on children these days, get them counseling, give them drugs. Most of the time, I believe kids just need attention, guidance, and patience from their parents. My son is 7, and he has made amazing strides just this past year. Some kids just take a little longer than others to grow up emotionally. And that's okay. School and his social world will teach him to reign in his sensitivity, or at least hide it when he needs to. Especially since he is so smart. Don't worry - he will eventually pick up the cues. One thing that helped my son tremendously, was that he made a friend that was in his grade, but was also at his emotional level. They have since grown apart, but it was a wonderful stepping stone towards his maturity.
Be patient, and always be your son's champion. Support him, and build his self-esteem, and he will be fine. Oh - and find a sport (maybe non-competetive, like through a YMCA) he's good at. You can't beat the team-building experience, and it will also build his self-esteem and confidence around others his own age. Good luck and God bless.