"Don't sweat the small stuff and it's all small stuff."
"If his heart is right toward God, the rest will sort itself out."
"If he chooses not to practice, his band grade will be an accurate reflection of exactly how much effort he has put into it."
I won't accept incomplete work in core subjects but music is a hobby, not a life skill requirement. Is it "small stuff?" I always look at what the bottom line principle is. While I don't condone sluffing off at any time, the deeper issue is the importance of your band-mates being able to count on you to do your part and not make mistakes. Practice is one way to get there. There is a minimum level of commitment he knew of when he enrolled. I don't think I'm being unreasonable by holding him accountable to fulfill that minimum level before he goes to play video games. Supervision and external motivation are still age appropriate for him.
Other goals: 1) ensure that our expectations are clear, 2) see that he meets his responsibilities to the family re: chores and shared labour, and 3) see that he completes his work and practice before play.
The little habits establish character in the long run.