My perspective as a HS coach in Chicago suburbs: We tell our kids in the late fall/early winter that the next team you will play for is the HS team. I think that is getting lost nowadays with the year round travel training. I expect our guys to be at our workouts if they can make it. I know there may be some times that guys miss due to a travel workout sometimes, but the communication needs to be there.
Of course, you are paying for the travel training. That is a factor. But truthfully from the HS coaching side, that cannot matter to us. If the HS coaches started catering to that and allowing all players to brush off what they want to do then how does the coach accomplish what he is trying to do?
HS coaches usually want players who want to be there. To be part of the program, to be with teammates during these workouts. Does it help win baseball games? Probably not. But it can have positive outcomes for chemistry, work ethic, trust in coach, etc.
Hopefully this coach can implement more planning going forward. We ask all players for their travel training schedule ahead of time so that we can come up with a schedule that least interferes. We implement workouts that complement those in the PE school weightlifting class and those that aren't.
Not sure if your coach is that organized yet.
If it was me, I would expect all players who want to be part of my program to be there as much as possible. It speaks volumes to a coach if it shows that a player may put another team first... ESPECIALLY if the next team the player will play for is yours.
We tell our parents too that your child will be doing baseball specific workouts with arm care, hitting, pitching, with our instructors (who are qualified)...For free!! Why pass that up? The answer we find out is because we are HS coaches. We all at least played college baseball but haven't been drafted at some point or are affiliated with recruiters or MLB scouts. We aren't qualified to make the kids better. And of course the other answer we get is because the money has already been spent for the travel training.
I know it doesn't sound like your situation is exactly the same, but your son has a opportunity to make things work with both teams as long as he communicates and priotizes what he really wants.