Thanks for writing.
The symptoms you have described could be typical of a few childhood disorders, but the fact that his mother is MR requires that this child have an evaluation by a child Psychiatrist.
It will be impossible for anyone to diagnose this or give you good recommendations over the internet - he needs to be evaluated in person.
When was he last evaluated?
Who evaluated him last time? A child Psychiatrist?
When you take a child for an evaluation you just simply tell the doctor his symptoms. You don't need to know what you're looking for -- as you aren't looking for it, the doctor is.
What do you do in the medical field?
If you answer my questions I may be able to help.
What type of professional evaluated him last time and conclude he was ADHD? And, if you are an RN then you know that ADHD is the classification - what was his type?