>>96260215
If we want to be fussy about rules, no need to throw the book. The ghoul cannot teach you. The lack of need of a teacher for physycal (or otherwise) discipline, though often houseruled that way, is purely left to the whim of the DM. RAW does say you need a teacher before you get any dot, and blood for the first dot.
Now go convince that guy to teach you without drinking his blood because your ghoul's is good enough