I'm thinking this is deeper then just baseball. At this age your son is resenting being told what to do and your husband is acting out over having his authority usurped by what he perceives as a subordinate. Thing is at this age the relationship between the father and son has to change. The son is now an adult and needs to make decisions on his own. The son is wanting this freedom, and needs to have it. The father does not want to let it go, but needs to. Its a tough thing to change the father son dynamic. My oldest is now a jr. in college and we have gone through it. Fortunately my wife, who works with youths, sat me down very early on and explained it to me.
Your husband needs to take a step back and let your son make his own decision and thrive or fail on his own. He needs to trust that you have raised him properly and he can do this on his own. Your job as parents has now changed. You now need to stand back and keep your mouths shut unless your son asks for help. Your job is to be there when he needs you or if he fails; to help him out of it. At this point, the only time you can really expect to step in and try to put your foot down is if you see he is making a decision that will ruin his life (ie drop out of school for a girl, etc).
The summer between HS and college as well as the when he comes home for the summer after his frosh year are the toughest times for this. You will eventually figure it out.