Being a demon means... being corrupted. Your soul, your blood, everything. The cure involves injecting purified blood — purified in the spiritual, holy sense, and in this case it would be my blood — into the demon every hour on the hour for eight hours. Then there's a Latin incantation to finish the job and bring him back to himself.
It's not any fun, for either the demon or the person doing the curing. But when it's over, it would bring him back to himself. I've done it before. It works.
It wouldn't get rid of the curse, though — the Mark on his arm that turned him into a demon in the first place. That's why, when he's himself, he'd rather be killed than cured. There's almost nothing worse that could happen to either of us, turning into the monsters we've hunted our entire lives. Demons have killed our family, people we loved, and almost destroyed our entire world. We protect people from them. Being one of them... I know it's my word against his, but it's not what he wants. It's all the Mark talking. And the lack of a conscience.