Letting him know you think it's normal and then ignoring it is the best thing to do. It's a part of him, it won't go away, he can only get more comforatable. The reason why it affects him so much is because a lot of people don't/didn't ignore it like you. You'd be surprised how people in school react to someone's stutter, they're gonna laugh at your face as hard as they can everytime making you stutter even more next time and procces repeats.