Have you considered letting him catch or pitch. Catcher is an excellent position for kids with ADHD. This is my son who is now a sophomore. They are involved in every play and decision.
I have to be honest. When he was younger I tried explaining to a few coaches that my son had ADHD and that he may need things broken down or repeated some understood others did not. Sometimes it takes a special coach that understands your kid and appreciates him.
After a while I stopped explaining to coaches and instead taught my son that when coached you have to make eye contact, ask lots of questions if you don't understand and if working together one on one repeat the instructions back. It took several years for the lesson to sink in. That has really helped him. A frequent comment now is that my son is a highly coachable player.
My son is like yours. He has ADHD and had reading and speech delays when he was younger. He's now an high honor roll student 4.0 GPA taking some honors and AP classes and is extremely self driven. It took a village when he was younger to set him on the right path. He had an incredible teacher that actually taught him how to study, outline and stay organized.
But I always told him that school was his job and I expected him to put forth his best effort always but if he was struggling we would do whatever we could to help him, tutors, extra help etc.
I also express a great deal of interest in his schoolwork and help him with studying and other school related things. The more positive reinforcement you can give whether through attention, time spent together and little rewards here and there the more he realizes you care.
I also held off on video games and cell phones for a long time.