Well, we can’t change our kids’ brains, so focusing will be somewhat of a struggle. Medication can help some. But you hit the nail on the head when you said he can focus on things he likes.
The key here is to make a list of times he can focus well, and things that are a distraction or break or prevent his focus. Then, use this information to formulate some focus strategies. Use his interests for school tasks he struggles to focus on. For instance, if he is really interested in cars, have him read books and articles on cars; have him write about cars. Use that interest to harness focus for non-preferred tasks.
If he focuses better when moving, allow him to walk in the back of the classroom during lessons or when reading, etc. If music helps him focus (does my husband and both my kids), then can he have music in one ear during class? And let him listen to music doing homework. (I know this seems counterintuitive — for me, music is a HUGE distraction and I couldn’t accomplish anything that requires focus, but I’ve seen firsthand in my family how the opposite can be true).
ADDitude Community Moderator, Parenting ADHD Coach & Author, Mom to teen w/ ADHD, LDs, and autism