I first of all want to tell you that you are not alone. I am dealing with this same situation with my Sons father and have been for the past 4 years the worse part is my son doesn’t like taking his meds so he wont take them on his own it is a constant struggle. I have even brought this up during court and its just pointless.. I spoke with his doctor and although it is helpful for him to be consistent with his meds she said it was ok for him not to take his meds at his fathers house. I dont know what your schedule is for custody its a little easier for me because my son doesn’t go with his dad on weekdays. But maybe you can talk to the nurse at school and arrange something to where the days he is with his father, they could issue his medication when he gets to school as part of his IEP.