I understand why your son doesn't want to play ball where dad is located. Kids who are playing a sport often are on a track within their area's teams and are trying to move up or onto different teams as they improve, and playing ball in another state will not do that for your son.....
I think your son needs to know you have his back beyond your saying that you support him, and he needs you to advocate for him more than you are. I realize that you want to "stay out of it and let him and his dad figure it out," and want your son to handle this now that he's maturing, but your son is still young enough that dad seems to be strong-arming him and saying "This is how it is." Yes, your son is at an age where he has to learn to stick up for himself and assert himself more with dad, but if the pattern has always been that dad dictates this schedule and will not even consider any changes -- your son must learn to stand up, and that means he has to be taught. Rehearse and role-play with him and have him talk directly to his dad, not his step-mom. Son should get on the phone with a written list of the specific reasons he wants to stay for baseball season and return before football practice starts. If he approaches dad with a long, clear list of specifics, dad at least can't say "You don't know what you're talking about."
I would expect dad, from what you describe, will still simply say "You have to do what I say." Then YOU need to talk to dad -- is your relationship so bad that you and the ex can't discuss this at all? Is that why he has his wife talk to you?
Before any of the above, find out the answer to your own question abut which state must handle any court actions. You really need to talk to a lawyer immediately, one who really knows custody laws and can find out about the laws in BOTH states if they apply. At what age in your state (or Texas if that applies) do children get to have a voice in court about their own custody arrangements? I have read that in some states, kids as young as 14 can speak before the court about custody and their preferences and in some states judges will listen to children younger than that! Could be younger in your state -- do you know? I am not saying drag it to court immediately, but you do need to be aware of what your son and you may have to do if dad continues to be, frankly, a jerk.
Unfortunately it's already late May and it may be too late to use the legal route for this summer, but you can plan ahead to stop the 45-day stay next summer.
It sounds like a new custody arrangement, where your son can't be legally obliged to go to dad for THAT long, is in order during the rest of the teen years or your son is going to resent dad big-time (even more than now) and honestly, he may end up resenting you somewhat for not being more assertive with dad and/or in court about this.