What makes you think he has ADHD? What are his symptoms?
is there a way to figure out if he has adhd or if he is just, simply put, stupid?
Realize that there is "stupid" and there is "doing stupid things". ADHD kids dont have the ability to think before they act- hence they do a lot of really stupid things. Most of them are actually pretty smart.
As a teacher I would look at tests and see where the kid would get the first questions right and then zone out, lose interest, and miss really easy questions later on. Of course, the child would fail the test (even when he might have known the material). This usually leads to the child getting highly frustrated and acting out. ADHD kids can concentrate when something interests them (like a video game). But try to get them to do homework - it takes forever. Many ADHD kids can carry on a very good, detailed conversation (age appropriate) about something that interests them. A child that is not very smart can't. I could go on and on. Do check out that link I first gave you. I can supply many more detailed links if you are interested.