I think stuttering gets worse when we make it a problem. Then comes the shame, fear, anxiety which sort of feeds the stuttering beast and then it grows on its own and sinks it's fangs into the pyche. The fact you are doing the opposite by proactively normalizing it and just letting your son be who he is, without judgement, is paramount to growing out of stuttering. The journey of self acceptance takes many of us decades to unravel. Your son is lucky to have loving parents who are "in the know" and he will no doubt live a happy and successful life - even if he continues stuttering. The irony is if you love yourself, the stuttering is no longer a problem, and does lessen or go away completely on its own.