My advice is, when he gets to the point where he knows more than you, or he is good enough where you have a hard time picking out his faults, get him to a pitching coach. I got my son involved with one when he was around 9 or 10. He was very effective, pretty good mechanics, but I had worked with him on everything I knew.
Don't be too afraid of change. Find a pitching coach who works with kids within the framework of a kids natural delivery. You are right in not wanting his style totally revamped, but every pitcher needs to tweak their delivery. That's why there are pitching coaches in the Major Leagues.
Talk with the instructor prior to a lesson. Watch the 1st lesson and if he is changing too much, move on to someone else. What you don't want to happen is have him develop bad habits that could be injurious that become so ingrained that it becomes almost impossible to change when he gets older.
I hope this makes sense. Good luck.