Biggest thing now is make sure regardless of what degree the stutter is, ensure his confidence and self esteem aren’t brought down. I had a speech impediment where I couldn’t say R and I still have trauma to this day at 25. Yes I am capable and smart but I mainly stutter because of my low confidence and self-esteem. I get nervous and I always think back to when I was younger and I couldn’t say my name. I am currently in speech therapy As an adult But when I was in speech therapy as a child I fucking hated it. I felt so stupid even then and my parents didn’t really know how to affected me because it’s really hard to know. My biggest advice to you is regardless of what the outcome is or the diagnosis just continue to build on your sons confidence. A good base of self-esteem and confidence will get you much further than just being fluent.