In this case, I'd use the Guild Member Background trait (he can use the guild library, etc but has to pay dues). In addition, using the downtime rules (DMG), he could learn a new skill such as Arcana, Feats such as Ritual Casting (Narrating his normal Feat selection as training), You could say he can't actually learn new spells upon leveling as normal without the downtime training with a mentor. etc.