He’s 20. While having ADHD means it’ll take a little longer and be a little harder to mature to self-sufficiency, that doesn’t mean he should be enabled to lean on the condition or have you provide excuses for him. The real world is never going to allow that. You either have to prepare him for the real world, meaning holding him accountable for his actions without hand-holding, or you need to arrange some kind of long-term dependent care for him (if he truly has no control over his behavior). There really isn’t a lot of grey area there.
That doesn’t mean your husband should act the way he does either. If this is how your husband has always been, and that’s just his personality, you might have to just let it go (since your son is 20 now, let them handle their own relationship). But if this is new, then he’s probably reacting to your approach by going to the other extreme. This is very common for people who are close but often disagree. Most likely, he’s concerned his son won’t survive the real world if his Mommy is always there to provide a crutch (I’m not necessarily saying that’s the case, just that he’s probably is thinking that and reacting to that idea).
In short, communication is key and may be lacking here.