I am in almost exactly same boat.
My son is 10 and his dad denies he has ADHD and says its just my bad parenting.
Despite multiple medication free trials that have shown a clear difference when my son is on / off medication.
My son only has his medication (Ritalin short release) for schooling. ie. not in the weekends or school holidays. I give it to my son in the morning before school starts and again at lunch time because his dad refuses to allow the school to and will not give it to him on his days.
My son feels very conflicted as he’s very loyal to both parents.
I would love to talk more about this as it’s (sadly) reassuring that there are other parents dealing with these types of issues.
If anyone wishes to chat further: firstname.lastname@example.org