The problem probably is that he can't tell that he actually needs to use the bathroom when he does. So he has to get into the pattern of going to the restroom even when he doesn't think his bladder is full.

He should attempt to go to the restroom during PE, at lunch, and other breaks. Even if he only gets rid of a little pee at those times, it might be enough to prevent an accident due to not feeling a full bladder until too late later.

I had a lot of problems with Diarrhea as a child (and as an adult). I had to learn to ask to go to the restroom and even if the teacher turned me down to just start running to the restroom so I wouldn't shit in my pants again. It was humiliating to have restroom accidents in the classroom.

One thing he can do is look at the times he is most likely to have an accident and then just make sure that he goes to the restrooms at that set time no matter what every day, even if he doesn't think he needs it.