Oh, another thing. I am guessing that you wouldn't have had your son tested if you didn't suspect something. It sounds like he is recognized by teachers as being very advanced, at least in math. Is he equally advanced in all subjects? I ask because you mentioned possibly just accelerating for a few subjects. If your son is advanced across the board, it makes more sense to skip the full grade/all classes.
Another possible concern is sports or academic contests, which you didn't mention. If your son is very competitive in either sports or you think he might like some academic competitions, you may want to consider the subject acceleration a bit more if you want him to compete with age-mates.
