Main thing: Don’t panic. Parents try to convince their children of all kinds of baloney but most kids are too smart to buy into it especially if one of the parents stays calm.
Also, don’t try to reason with your husband. The stress of the divorce has triggered an underlying mental condition; you could be the most persuasive speaker on the planet and he’d still ignore you because he’s not thinking clearly.
While you sound like you’ve started to heal, he’s still stuck in a painful loop of anger, remorse, embarrassment, shame, humiliation, and all the other emotions that come with the breaking up of a family.
For whatever reason, he has fixated on meds and ADHD as the locus of his uncomfortable feelings. He could have seized on anything related to family life: pets, mealtimes, chores, sports, church, but for whatever reason his brain grabbed onto this topic.
Keep reminding yourself it’s not about you or your authority; it’s not really about your son either. It’s a private war between him and his demons.
See what strategies you can come up with to work around his problems while maintaining cordial relations until he comes to his senses.