I would say to look at how things are being handled between the two households. Is there bad talk about the other parent? Is there any tension or anger? Children pick up on alot more than what we realize.
Then, I would suggest that your husband needs to take the primary role (this does benefit the whole family, especially as the child gets older) and counseling at school could be the next step. I say this because, usually if the school is recommending counseling, therapy or testing for ADD/ADHD this benefits by backing the parent (s) up who want to proceed with getting help for their child.
If the mom is still not willing to try counseling and so forth, you can proceed with other legal options. Possibly suggesting it to her as "just a trial basis" to see if it helps him in school.
Parenting consultants and/or mediators may need to become involved. They are less expensive than court and they make the decisions in the childs best interest.
Definately needs to be Dad putting this into motion, not saying you can't support him, but the school, parenting consultants/mediators and if it ever makes it to court - the judge will only see you as interferring between the parents of the child.
I just went to court yesterday to do a simple modification of visiation, something my daughter's father and I should of been able to work out on our own. But because of the extensive involvement of his wife, there are now some specific ground rules laid out regarding her to more or less butt out.
One of my issues with the stepmom is her involvement in my daughter's medical records. This is my daughter's privacy that she is invading and it bothers me alot, because she goes to public places, makes copies and mails them to different clinics that my daughter goes to. This is not her role. Be careful of just how much you are involved.