Oh wow. kids with ADHD can a very high level of anxiety as a co-disorder, and that is what is beginning to happen to him. When you are anxious, you can't think - among other things.
And, you are not helping the situation. You need to do things with him in small segments. No more then several minutes at a time. But more importantly, you need to make sure the school knows he has ADHD and compensates for this. If they do not, they can ruin him.
Also, do you attend his therapy sessions? You really need to learn all you can about ADHD. You are going to be his main and perhaps only advocate for a long period of time. You really need to understand how this affects him and how learning can be accomplished with this. If you have questions about how to help him, feel free to contact me on the other site where I am also the CL and deal with this alot. https://
www.medhelp.org/forums/ADD---ADHD/show/175