I would advise you to firstly let go of the intention “I don’t want my son to stutter.” That is probably the worst thing to tell him as it builds even more insecurities. The best thing you can do is work on his confidence, tell him he’s good just the way he is and that he’s capable of anything. There are lots of people who stutter that are successful, confident and happy. If you want to be the best support for him then have trust in him, build his confidence and see him with accomplishments even when he can’t. I’m not quite sure what the height has to do with anything and can’t see why would that bother anyone. Best of luck!