tbh the way i did it was firstly doing some stealing in the kitchens so i can skip class later on. make sure you have study of malediction (your first interaction should be on that class w/ him)
for me, he's either in the kitchen(skipping), in the gardens or in the library
having a lot of interactions w/ him is very important!! once he starts talking about malediction, you're on the right route
interaction goes like this(for me):
wed: lunch & class(SoM)
thursday: kitchen(skipping), library(freetime) and lunch again
friday: class, garden(freetime) then lunch again!
hope you get it!!