I am not a parent, so I can not offer any first hand experience.
I was, however, a year ahead in school like your son. I always loved it, I think I got extra attention as the "baby" of the class(I think there may be a difference though with boys and girls), but there were some times it was difficult. For example, I never had friends my own age, they were always a year older, and they went through things earlier. Sometimes I felt a little left out. When my friends were boy crazy, I still just wanted to play dodgeball. My friends were allowed to go out with friends or dates before me, they could dirve before me, etc. The one thing I have always liked was that I was going through things earlier than girls my age. I had my first kiss and first dance with a boy when I was 10 years old, in sixth grade (much to Dad's dissaproval, lol) I never missed out on anything major, and I had a wonderful chilhood, but I definitely understand your concern. I know my mom would agree with your choice. She always felt that what was right for me was that I be academically challenged, but the day I move to college she actually told me that she felt that i missed a year of my childhood, and that if I wanted to stay home another year I could. I was 17, and she was scared, but it worked out for me. She still says she wished she had me for that extra year.
I wouldn't lie to him though. I would sit him down and explain your concerns, tell him there is nothing wrong with him, but that you feel he may be happier socially. Ask him what he thinks, make sure he feels that his opinion is valued. You may find that he agrees with you.