If he wants to have a future career in coaching, he really should leave. Same reasons you don’t hire friends/family to work for you. If they do poorly, they’re only there because of you. If you come down on them the same as you would anyone else, you’re not really holding them accountable enough because of the relationship. If they do well and get promoted, once again it’s because of you and your connection with them. No matter how he does, he will never truly get a fair shake.