I'm guessing the curse feeds on his magic, but he's slowly building up a tolerance. Hence able to level up. The artefacts could help in theory, but he's not getting an xp boost for them. It would be interesting If we had to feed him an artefact for each level after a point.
As is, the relationship isn't really explained aside from "literal bomb feeding on magic". I'm personally very curious about how much of the truth he's telling. Some people think he's secretly evil, but that seems a bit too tacky
