Hi M.! Just to add some light at the end of the tunnel: My son also has severe ADHD, but was not diagnosed until he was in 3rd grade. His inability to pay attention to the teacher was the reason he struggled in math prior to his diagnosis. The 1st step is realizing that a child has ADHD and treating it. It's wonderful that you've already gotten that far!!
Have you thought about the time of day when your son is in his math class? Is his medicine strong enough? Is his medicine extended release? Maybe the medication is not working if his class is early in the AM or late in the afternoon; the medicine may not have had time to release into his system in the morning or is already out of his system by class time in the afternoon. Also, have him tested for anything you might notice he has symptoms of. Have his eyes checked, have him tested for dyslexia. There could be another medical explanation. Talk with his doctor about newer ADHD medications even. The advancements they have made are amazing!
And, it could be that having ADHD has nothing to do with his problem at all! Lots of children struggle with math! My 10 year old daughter included! My suggestion would be to talk with your son's school counselor and find out if there is a TEACHER there that is willing to tutor him privately. There are teachers at my daughter's school who tutor and I bet you will find a willing teacher in your son's school as well. And I agree 100% with the previous responder who talked about the computer games! They are WONDERFUL! This was how my daughter learned her multiplication facts!!! Making math fun is key!
The light at the end of the tunnel is this: It gets better! My son with ADHD is now a sophomore in high school, taking advanced placement classes and making grades I would have NEVER expected from him when he was in 1st grade (Or 2nd or 3rd, for that matter)!!!
So don't be discouraged! You have already done the MOST IMPORTANT part: You have recognized the problem and are trying to find a solution! Best of luck!