8. He is starting to express reservations about doing activities with other children that would require an amount of time that would prevent him from doing it (i.e., sleepovers, summer camp).
This point raises a red flag in my mind. When you first described the activity it sounded normal enough, but then when you said he's still doing it a 11, it gave me pause. But your point #8 seems like a real OCD sign.
So, where do you go from here? Maybe start by talking to your pediatrician if you have a good relationship with one.
The way you've described it sounds like you have a level-headed perspective on this, which bodes well for you in charting the right path through this situation.