Play him up!! If the coach gives you ANY assurances whatsoever that he will at least give him an opportunity, he will benefit for all the reasons everyone else gave. Our related experience:
A coach from a team in a local college level [23 and under] wood bat league invited Junior to pitch for his team this past summer, after seeing him at a local showcase over the winter. We cleared it with him, that Jr would still be on the Legion roster, in case he wasn't getting enough innings, or wanted some ABs, since that part didn't look likely with the college team [he was still 17 YO, only HS kid on his team, only 3 in the league]. Decided to basically forego Legion, for many reasons [weak league, even weaker team, many of his friends not playing, etc]. Result: ended up pitching regularly in the college league, was the team's most consistent pitcher [not best, mind you, (they had a kid that has been drafted 3 times) just consistent: 6-8IP, minimal ER each outing]. He pitched a CG 9inning shutout, pitched 6 innings of 2R relief for the win in the first playoff game, and started the 3rd playoff game 3 days later, losing 1-0 [all this in the span of 8 days, with 4-5 pitchers with college experince on the team]. Several college coaches that were in attendance to watch some of their own players, saw him pitch and are now in regular correspondence with Jr about going to their school. The team that won our league picked him up for the NABF World Series in Toledo, and he pitched equally effective out there - a great experience for him, all the way around. This would NOT have happened if he had played Legion ball.
Due to some injuries, poor attendance by some guys, etc, he ended up playing a LOT of 3B, too. Got a decent number of ABs, which was an added bonus.
Our thought process was that he would be challenged, and he would learn from guys that are playing in college.
If your guy is good enough to make the team, let him go -- the challenge will benefit him, IMHO.