When your child falls down and hurts himself, you acknowledge it. You give it a name. It's okay to do it with stuttering too. Something simple like, "Oh, your words got stuck", will do nicely. In general, for now, without asking him to do the same, slow down your own talking around him (kind of like Mr. Rogers) and use a slightly melodic tone. You can also use puppets who do the same thing when the situation suits such an interaction in playtime, for example.