I second most of these comments about what you're doing. The one thing I'll say is most SLPs are not trained in handling stuttering and even the ones that claim to be get it wrong half the time. If the therapy he gets doesn't seem like it's helping or the focus is entirely on telling your son to speak differently to not stutter, have some backup therapists ready. Don't be afraid to switch if need be. He should learn that stuttering is ok and he can say everything he wants to say, stuttered or not.