First thing: Don’t panic. Parents try to convince their kids of all kind of bulls**t but luckily most of it slides right off them.
Explain to your son that sometimes people who love us aren’t always able to find the words to express their feelings and it makes them act in strange ways.
Your ex isn’t doing this out of any authentic concern for your son. He’s obviously neck deep in grief, shame, guilt, denial, frustration, and regret over the divorce and it has short-circuited his logic. He can’t listen to reason because he’s just a big ball of angry sadness.
Something about the whole topic of ADHD has has triggered this fear reaction and made him fixate on the ADHD. Was it the last thing you guys dealt with before the divorce?