Actually what you will find is most really good tournaments have a combo of HS and college fields. From when I used to book tournaments for my team, the sooner you get your entry fee paid the better chance of being on a college field. That is not an exact science but seemed to work.
I think you / your son should thinking how far from home he is willing to go to college and adjust from there as well. If he only wants to be 3 hours from home, No need to go to Perfect Game Juniper probably (although there will be those colleges within your radius there), If he is open to wherever, then being at those kind of tournaments really can help. Target where the colleges he is interested in go watch, make sure he tells them when he is playing and go from there.
From a 15U point of view, not a lot of colleges are watching unless your son is in very upper echelon, 16U, still upper echelon (top 20 in state of Michigan in his class). 17U is the key summer (between junior and senior year). 18U is for almost everyone of them after they graduate and they are already set where they are going. 17U and 18U are almost always not separated at tournaments.