Therapy really helps at a young age, atleast from my experience. My stutter almost completely went away when i first went to therapy around the age of 10, but then it came back and we couldn't afford therapy again so it kept getting worse to the point that i just write everything now instead of speaking. There were other factors to it in my case as well like the death of my father, a lot of bullying and intolerance etc. Even if you can't afford therapy, just treat them like a normal person, wait for them to finish their sentences and never make them feel like it's anything weird. Make sure he's not getting bullied as well and allow him to express everything to you guys. Good luck, You're an amazing parent.