6'2 160 is not small at all for a freshman. In fact that is very big. I'm not sure where you would be seeing a lot of kids that make him look small.
But once you enter HS age doesn't matter anymore. All that matters is graduating class. No coach is going to care about his age. There are plenty of "young" kids with summer birthdays. Would being a grade lower help? Yeah probably, but it's just something you deal with at this point.
What your son needs to do is decide which position he wants to play and either tell the coach or find a new team. I'm assuming he plays SS on HS because he's the best player. I'm assuming he plays OF on travel because he's either not an infielder or that is where the coach sees him playing at the next level. That is for him to decide, but to say he can't excel is an excuse. He plays SS for 3/4 months and then OF for another 3/4 months. That is plenty of time to get used to each position, which I'm assuming he knows how to play both at this point. Most of the improvement is made in the offseason anyway.
For now 6'2 and mid 80s will get a lot of attention on the mound. Unless your son kills the ball and has a smooth glove I would just be sending him as a pitcher to the showcases.