Who has the right to make medication decisions.

I am the residential custodial parent of my 13 year old son who was diagnosed with ADD about 4 years ago. He was given a prescription which we have only given it to him on school days so that he can focus. Because it tends to decrease his appitite and limit his ability to, as he puts it, "have fun", we have never given it on vacations, holidays or weekends. Even though his non custodial father was originally against medicating him, he has recently entered into a marriage and his new wife is often bothered by my sons tapping, excessive talking, high energy level. To make matters worse, the have recently had a child and my son is feeling very displaced and rejected. My sons new mom wants him medicated all the time and his father is now trying to get a prescription from the doctor to medicate him on the weekends. His father has never had the medication before because he does not have his son on school days. Can I as the CP tell the doctor to refrain from giving a script to his dad?