you know, chances are socially its not going to make any difference. i only WISH my parents had chosen to let me go ahead; at least it would have gotten me out of the hell hole that is high school a year sooner.
kids were vicious to me. i was different from them; i did my work, got along with the teachers, the teachers were the reason i got up in the morning. i didnt drink, didnt smoke, didnt party, nor was i invited. i was a music geek, not a sports athletic person. i did not fit in. it wouldnt have mattered if i was 2 or 3 grades ahead; i was never going to fit in, no matter what happened.
i had my own little group of friends. we were basically "the leftovers". we didnt really have a lot in common (even they were smokers/drinkers) but we had common threads between us : we respected each other, we were there for each other through the drama, and we were all rejected by everyone else. we knew what it felt like, so we strived to never treat anyone else that way.
my point is this: educationally, your son is going to be bored out of his skull in a few years, if hes not already. kids that are bored, CAN and sometimes DO become the class clowns, the troublemakers, because they are bored and have nothing better to do. they can also become "lazy" - not really having to study can set them up for failure later on as they HAVE to study, but dont know how. most people dont realize how important it is to struggle for something, and how frustrating it can be to NOT have to struggle, and hit a wall when they do. i know this also from experience; i never had to study for anything. the hardest thing i ever had to do was read a novel or two (because it was hard for me to retain what i was reading sometimes, because i read too fast, or my brain would wander LOL)
so when i hit college, i didnt do very well (or i didnt feel like i did) because i didnt know how to study in the first place. nothing can prepare you better than experience.
anyway, my suggestion, or my answer to your question, is to let him go ahead! let him excel! let him be pushed a little. if he flat out fails, you are open to the idea of resuggestion. talk to your school about the "new" education plan going around: individualized student learning plans. my old school is in the middle of change that will bring them to the point where they can have students of different levels all excelling with the same core materials, yet supporting each student at their level. some kids can do a project on a more detailed, difficult level, and other kids can get a break, yet still get equal credit on the work they are able to do. this would help your son to excel no matter what grade he is in, but it takes a lot of work and training by the teachers..... and i dont know if all schools/districts are ready and willing to take that step yet.
anyway. just do it! :) socially, it probably wont make any difference, because kids are kids, everyone is going to get picked on for something, and i would rather your son be confident in his learning, and his gifts, than to be a target because of boredom or just because hes advanced or something.
anyway. good luck with your decision