Keep the idea of him needing to consume livers and spleens to charge mana, and get rid of the collar, it is cheesy imo, your character is badass, slapping a collar there hurts your character's image.

Here is a suggestion that might impress that GM,

Have your character develop a weakness to human blood, so even though he needs to feed on liver and spleens, the sight of blood weakens him and therefore can put him in a vulnerable situation. This should automatically limit the risk of your character going beserk and tearing up students for livers, since that much blood will make your character a class A wuss.