Poor little guy! That's got to be hard for him to deal with.

It sounds like he is not recognizing the feelings of needing to go to the bathroom, like beck said. I agree. I would try to get him to go to the bathroom at set times every day, even if he thinks he doesn't need to go. Maybe first thing at school, at lunch, etc. Maybe his teacher can help him remember at those times, maybe have a timer for him. Though not sure if that's too embarrassing for him.

Then do the same at home, set up certain times to be bathroom breaks. Have him shower or bathe daily to help with any residual smell. I remember a boy in 5th grade who smelled. The other kids would tease him all the time. I didn't tease him, but I still feel badly that I didn't stick up for him, just stayed silent.