Some thoughts...
yeah, "antifreeze". Some life forms have a higher glycerol content in their cellular fluids to lower the freezing point.

Many mammals withstand artic temps, like whales. Of course, their mass is much larger. But, perhaps he has an inordinant amount of brown fat, which is used in thermoregulation. Supposedly humans don't have much, especially adults. But, it's got a high metabolic rate, and might be producing more heat?

Lots of cold weather animals also have counter-current circulatory systems: warm blood going to limbs passes cold blood coming back. Helps to retain core temp. It's possible he's just set up wierd...

Either way, I really hope this guy says his body goes to science upon his death. But by then, I'll probably have forgotten, it won't be published, and the world will stay ignorant of the root of his ability.