There are many other threads from parents like this one with good advice. You can reach out to the NSA or stuttering foundation to see if they can recommend a therapist in your area. Finding a great therapist is the hardest part, but a great therapist can make a big difference. If he is straining I would find someone. You can also reinforce to him that it's ok to stutter. He doesn't have to push his words out. An SLP should be able to help with that.