You should treat the subject like it's not an issue at all. You don't go around announcing your problem to the world, but you don't try to hide it either. So the best thing might be to not tell him directly, but to let him find out on his own. Do you wear protection, disposables or cloth with plastic pants? Leave a bag of disposables or a few folded cloth diapers on your bed in plain sight in the morning. Then invite him over after school (or after a movie or something on a weekend) and take him to your room. He will probably notice the diapers and ask what they are for, and you can tell him that you use them for bedwetting and that your mother must have done the laundry or gone shopping and left them there while you were out. Then just put them away like nothing happened.