I'm assuming that had he been offered the chance for his HS team that they would've accepted it but since it wasn't offered, they looked elsewhere. If I was in that position and my son wanted to play this Summer and it was not offered by the HS that I would've done the same thing.
If you're that age as an underclassmen, he would probably be better off using the Summer to advance the knowledge of the game moreso than anything, IMO. I haven't played fresh. BB in almost 20 years, but there are things he's going to need to know as a Soph. that he might not know, whether it be defensive positioning, footwork, reading the pitcher when standing on 1B, increased signs, etc. It should be considered a bonus if a kid ends with up a new coach over the Summer that helps a player improve a skill.
Most importantly though, it is more about the coaches and what they are trying to find out what they have for next Spring.
Obviously though, they are likely to improve more practicing than away from the diamond. I'd rather seem them doing more advanced drills than the same drills they've done from March till June.