I agree with others here. My advise would be that Son talk to new coach, stating that he is fully committed (assuming he is) with making the HS team priority #1 when coach thinks it is appropriate. Present the routine that he is doing with the travel organization. Ask for his advice.
Lot's of possible scenarios can come from this - perhaps not all of them optimal. At the least, coach will know he is working hard, serious about baseball, respectful of the coach and willing to put team first.
But you and son have to be willing to accept if coach decides he wants him with HS workouts as priority #1 even if you know that means a bit less quality and specificity on the workouts. As others have touched on, there is value in team bonding and setting example with the HS teammates with your work ethic and loyalty. Hopefully, a good plan can be hashed out with the coach. If not, your son can probably add some of the key missing elements of the workout on his own until HS season is over and he can resume with the travel org.
As a HS coach, in this situation, during summer and fall, I am open to the player getting most or all of his work in with the travel org if it is clear he will have the opportunity for better competition and more individual attention. Come winter, the expectation is team first. Travel orgs certainly understand that at some point, it is time to shift priority to HS for the year and that point varies from HS program to program. There is almost always a solution that gives the player the optimal chance to develop while maintaining team chemistry. It's not always easy to find the right balance though.