This question gets asked every year. Once I looked at PG WWBA 15U teams of large organizations, there were a number of kids on those teams playing down from their grade level. Most were not committed, whereas the kids on the organization's 16U teams were. My conclusion was, the organization will say "this team will mostly play in the older age groups" or "will play in national events" but what they're really saying is "you're not good enough for our 16U team" or "you're not in the D1 recruiting pool now".
My impression is:
If he throws 90+ (or whatever the equivalent is for hitters) at age 15, it doesn't matter what he does.
If not, aim to be playing 17U the summer before senior year, since that's where below-D1 recruiting takes place. That summer, you don't want to be playing 18U or 16U. Before 17U, play on the team where he will get the best coaching and playing time.
Playing up (above grade level) shows "I can hang with older kids". Playing down (below grade level) might make people ask "why are you afraid of playing with kids in your grade?"