I had a hard time with this too, make sure you pick all of the classes he's in (history of magic, healing, malediction) and then what worked for me was skipping class once (this will mean you have to get your bad stats up a little) and when I skipped once/twice and I went to history class and clicked barnaby, I got a chance to work on a project with him. After this, I made sure to be in the library (either to read or do the project with him), go to the garden (to help him with fanfiction), and at nap club to catch him there too. After that you should get his ending :)