Show him that it’s okay to stutter, don’t give him negative associations with his speech. For example, when he’s speaking, look at him with a loving smile and wait patiently until he’s done. Respond to the content of his speech regardless of the stutter. If he’s making a joke, laugh at the joke. Dont draw attention to it with the intent of fixing it or correcting it. Also don’t ignore it and treat it as a taboo subject. Just treat it like another beautiful part of who he is.