What a difficult and stressful situation. Trying to juggle all these competing issues must be a terrible strain. Let’s see if I can help a little:
First things first. I always start with the person who is the most vulnerable. In this case it is your son. You are correct that it is important to deal with his issues now, before the teen years. ADHD is not a tragedy. It can be managed. Kids with ADHD do grow up to be successful in life. But they usually do need help learning how to manage their own thoughts and behaviors.
It seems to me that there has to be some recourse for you for getting treatment for your son. Talk to the special education department in your school system to see what other parents have done when divorced parents don’t agree. Consult with a parent advocate for parents of kids with special needs about your rights. (Your school system should be able to provide a list of qualified advocates.) If the problem for the father is that the program you’ve investigated is Christian-based, do consider alternatives. There may be equally good programs in your area. Ask the school counselor for advice. Further, I hope your son is benefiting from an IEP (Individualized Education Plan) through the special education program at his school. If not, do get that process going right away.
Meanwhile, reassure your son that you know it is hard to have ADHD, but that you also know that he is a smart kid who can start to figure out how to handle it. There are some excellent books for kids that speak to the issue. Explore the book sellers’ websites for ideas. Perhaps the two of you can read them together.
At 9, your son is old enough to understand the facts about your boyfriend’s illness — and to know how hard the situation is for you. Remind him that part of growing up is to take other people’s feelings into account as much as he can.
You can’t make your son’s father cooperate. What you can do is help your son understand that there are often different rules in different situations. The rules in the classroom, for example, are different from the rules on the playground. There may be rules at your house that are different from his father’s rules. That’s okay. Another part of growing up is figuring out what to do in each environment. Talk it over with your son. I bet he already knows what to do where. Emphasize that you have faith in him that he can do it.
As for the stress on your boyfriend: Please consider relieving him of all responsibility for being an active co-parent. It’s enough that he care about the boy and respond positively whenever he can when there are natural interactions. That’s all. It will reduce his stress if you take on dealing with the school, the child’s responsibilities at home, and discipline.
Get out of the middle by refusing to be there. Don’t ask your boyfriend to parent. Make it clear to your son that you are his responsible and loving parent and you expect him to go by your rules while at home and at school. Instead of fighting with you ex, call in some help from the school and, if necessary, a lawyer.
I wish you well.