I can’t recommend talk-therapy enough. Ironically my partner is a talk therapist and there’s a ton of overlap between our jobs. Lots of negative (and false) thought spirals that would really benefit from being heard by a professional. But if that’s something that’s troubling you, I think you can really value and put an emphasis on creating a childhood environment that helps prevent him from forming that stigma against stuttering. He’s too young to have seen the Billy Madison stuttering joke, or for a peer to be confused by his speech yet. One day those will happen, and that’s when the parenting skills and talk-therapist really come in to help you as well as him handle those moments. The other commenter here hit the nail on the head for some more specific tips. Trying to ‘help’ him become more fluent does the opposite, it reinforces that stuttering is bad and he should avoid it. It’s bad and it’s WORTH being the butt of a joke and he SHOULD be hiding it. Let him stutter it out and it won’t be worth making fun of. The only reason you would tell him not to stutter is if stuttering is something that makes you bad, right?