Right now, the best thing to do is to don’t make him worry about like like saying stuff like “Take your time” “Start over, and talk more slowly” At this age, it’s normal for children to have a developmental stutter as their brains are still learning how to talk. Don’t try to correct it, as it will make him notice something is ‘wrong’, and he’ll probably start feeling insecure. But like you said, get him a speech therapist, and be careful with the one you choose because a lot of them aren’t trained with disfluency, so it could potentially make it worse.