If your son had ADHD, then he should be seen by a mental health therapist for this condition. To that end, you should discuss this conflict with that therapist as it has direct bearing on your son.
If your son isn't being seen by a therapist as he should be, then I would advise that you and your husband consider family or marital therapy to resolve these issues.
Keep in mind your child has ADHD and not everyone is "equipped" to deal with this in the best manner possible. Your husband is correct in saying he is doing the best he can because he is no expert in regards to dealing with a child with ADHD. I can relate to your husband because I deal with a stepdaughter who has ADHD. It is not easy to deal with and it can wear on one's patience.
You and your husband would definitely benefit from therapy. That would give him and yourself the proper tools and direction to deal with this. The child should be in therapy as well.
My daughter also has ADHD, and my fiance had the same problem getting mad at her. I got her on meds and went through therapy with her. I learned some really great discipline techniques in therapy that really work for kids with ADHD. I came home, sat everyone down and explained the technique. We then practiced and did some role playing so that everyone was on the same page about what was to be expected. My fiance kind of rolled his eyes and was annoyed by the whole thing, but once he actually got the chance to implement it and see how well it worked, he was impressed. Habits are hard to change and it won't happen over night, but try something like that. Good luck!