When is your son's birthday? Is he old or young for his current class? If he's old for his current class, the gap will be less with his newclassmates than it would be if he was young.
My grade-skipped daughter is 9 with an end-August birthday. She's tall and keeps up with her classmates during PE and she enjoyed being on the elementary school soccer team last year. She won a prize in a school-wide 3K footrace last year at 8 (4th out of 20 or 30 kids who chose to race). So she's fine.
A couple kids in her class are 2 years older than her because of the birthday thing. I see some social maturity differences, but she gets along very well with her classmates overall and is happy socially.
My double-skipped son has an April birthday. The age difference was most difficult for him in 8th grade (homeschooled in 9th; back in school now), but seems to be less bad now in 10th, presumably because he's maturing in a way he wasn't two years ago. He's strongly considering taking an extra year to finish high school (via a middle college program). It'll allow him to do a lot of college courses in high school and will let him be 17 when he applies to colleges. FWIW, I don't see a year as being a much of a gap after 17 or 18 or so. DS also seemed to keep up with other kids in PE when he was 12 and they were 14, and always placed with older kids in swimming class (always kept up with the group during laps; sometimes was first to finish). He didn't join any teams then and hasn't now, but I suspect he'd do well in any individual-based sport.
So, my anecdotal evidence says, "It depends." When I was in high school, there were always lots of 8th - 10th graders on the track and ski teams who were as good as the juniors and seniors. There were also (fewer) younger kids on the varsity sides of team sports, too. I was on a lot of teams (3 per year), and used to see the same kind of thing on teams from other schools. So this is somewhat better than anecdotal evidence.
Overall, it just depends on ability. If the goal is fitting in rather than getting an athletic scholarship, fitting in will be a lot easier.
