My husbands nephew was in his 39s when he left home, although at that point he had a girlfriend who shared his room. Eventually they decided they were able to have their own rental and moved out
My own husband was in his fifties when we married and he was still living at home... Possibly once he sees he can afford a place of his own perhaps he could kind of do a gradual move, if the rental allows this, you know like paint a room here, gradually move in things from his room, like a toe by toe thing.. Once he begins to actually enjoy being there... He may move in full time...
Parents-in-Law. What do/did you call them?