What do I do if my ex is not following the court order for visitation, transportation costs, contact, etc?

First of all, one needs to know what type of parenting agreement and custody you have here.  Althoughit is obvious that your ex is the custodial parent for your son and you have visitation rights on vacations, etc.  But what about the right to make decisions on his behalf?  Health, school, etc.?  Is that right yours as well as a joint custodial parent?  Then you need to start asserting that right by obtaining progress reports on school and figuring out what is going on.  And if you need to modify the agreement or do anything to enforce it it has to be done in the state in which the child resides, so not yours at the present time.  If your child is failing and needs summer school that does not necessarily negate his coming to stay with you.  You can work with the school to find a program in your are for him to attend that they will give him credit for.  But you need also to step in and see why he is failing and get him help.  Good luck.