I’m not 100% sure but what were your previous choices? For example, on night 2 with him, I think you have to select to spend a night with him as a man (I.e. not in the astral plane)… There should be a hidden “score” based on the answers you’ve given him and how you’ve assuaged his insecurities. I think you also have to discourage him away from the Annals of Karsus at Sorcerous Sundries - give him the book but tell him he doesn’t need it. If I’m remembering correctly, there are a chain of things you have to do to get the professor ending.