This is a very common behavior for ADHD children. When they have had people for years telling them that every thing that they do is wrong - this is the end result. Its not that he is in denial as much as he is beginning to give up, because he is realizing that nothing he can do is going to help the matter. Yes, he does need therapy to help him. So do you, so that you can understand what is going on and help him. this is a very basic reaction. The fact that you haven't recognized it means that you have not gotten some of the information about ADHD kids that you should have been getting. Not your fault - your doctor's fault, definitely.
Until, you can start getting more info from your doctor, I suggest that you read, "The ADD/ ADHD Answer book," by Susan Ashley. It will really, really help you to help him.
Have you tried him in therapy?