Dad won't give ADHD meds

My son is 7 and took ADHD medication last year and did great. He improved dramatically at school. For some reason this year his Dad refuses to give him medication. He is having problems at school. A psychologist, a Psychiatrist, the Pediatrician and the Guidance Counselor at school have all recommended he be put back on meds and his dad still refuses. We have joint custody at this point, with his dad a week, then me a week.

I am going to file for a modification of custody so that I can gain primary custody, but how will this ensure that his Dad gives him the medication?

You will have to present the issue to the judge and seek that medication dosing be included in the Order.