family sharing aps and ID conflict

My son's phone was logged in under his father's apple ID for several years. Since we have family sharing we never noticed...until now when he got a new phone and tried to log in under his own Apple ID. The gaming aps he uses most were all purchased and logged in under his father's apple ID, so he had to log in with dad's ID in order to get them to load onto his new phone. So He wants to pay for the in ap purchases etc. but it is still part of the family sharing plan. SO If he leaves the family plan he loses the ap AND the in-app purchases??? If dad leaves the family sharing plan does the ap leave with him? Would my son have to purchase it again anyway and still lose the In-App Purchases? Is there anyway dad can transfer the ap to his son?