Makes me think of DNP, a substance which acts to dissipate the protein gradient in our mitochondria, wasting the energy that the mitochondrion would otherwise use to make ATP. The wasted energy is turned into heat, giving its users an elevated body temperature and characteristic sweating. It'd be the perfect dieting aid were it not so toxic.

There are proteins in the human body having the same effect, performing some minor functions. Maybe he had a mutation making those proteins very prevalent, essentially making his metabolism very wasteful. Although I don't know if such a sustained fever wouldn't be deadly.

https://en.wikipedia.org/wiki/2,4-Dinitrophenol https://en.wikipedia.org/wiki/Uncoupling_protein