If the child's mother is his primary guardian, unless you have reason to believe the child is being harmed and are prepared for a legal battle to challenge her custody, then I am afraid that the conclusion is-- there is nothing you can do. If you have expressed your opnion to her, and she chooses not to use it, you can either legally force her to use it -- or keep quiet. There is really nothing else you can do. And if she is working, providing for the child, making sure he goes to school, making sure that he is getting proper supervision, feeding him, providing medical care, and not physically and/or mentally abusing him -- then you don't have any grounds to challenge her custody. It sounds like you have a difference of opinion in terms of how to raise a child-- which, because she has primary custody, means her opinion prevails.
You are in a tough situation. I have been there in the past with my stepson. What does his father think about the whole matter?? Tough as it is, HE has to be the one to act (with your support) since he is the parent. And he might want to speak to teachers and get their opinion directly. We found that a few teachers were definitely on our side and saw what we saw.
You certainly are in a difficult predicament... But this child is lucky to have such caring people in his life. While people are correct in saying that "Mom" has the rights and final say - so does his father (in most cases).
If I "heard" you correctly - the father is concerned too. If that is the case, then you have some wiggle room. Obviously the best place to start would be to express your concerns to his Mother.
Whether or not it is ADHD? That's a tough one. It is too common for children to be placed on medications after only having answered a few key questions. In this situation, you might want to have your stepson receive a thorough medical evaluation as well as psychological testing.
There are far too many "other reasons" that children display symptoms that are consistent with ADHD.
Good luck. Feel free to contact me if you have more questions.