Bosco limiting himself to feeding only 4 times a year could potentially shorten his lifespan, given it's already made him anemic and has left him without much of the vampiric enhanced strength he'd have if he fed normally.
Not only will Bosco likely outlive Makiko by centuries, she also needs to stay a virgin for him to feed on her blood.
Alternatively, assuming Makiko's willing, he could always turn her. But it doesn't seem like the sort of thing he'd do, since he was trying to become human (or as close to it as he could get) for her sake.
