Have him at least get to peak rank 1 with 5 max proficiency skills before the dungeon itself ends instead of him choosing to leave. Otherwise people will be upset at not min maxing.
Hitting Rank 2 also works, if you want the dungeon to be a reoccuring thing you can have it temporarily force him to leave but with a key or something to get back in later on, for example when he hits peak rank 2 etc.