How long was he in the hole?

Snow isnt great for survival over the long term. Now, add superhuman cannibal power and it could work.
The best part about this movie is that it's just fun which is why it doesn't matter.

Yeah, there may have been a bit of snow down there, but I didn't see enough of it to sustain a man for a full month. Now, the "superhuman cannibal power" theory could make sense, but if you pay attention to the scene closely (specifically the moon cycle), you'll notice that Boyd doesn't actually begin cannibalizing Reich for quite a while. Rather, he spends at least a couple of weeks just lying there—a mere mortal, who, already having sustained severe physical injuries from his fall, is now exposed to the brutal elements of his harsh environment, facing intense cold, and basically lacking any food or water.

So it's actually only after those first two or three weeks that Boyd succumbs to his fear and weakness, finally deciding to give some of that tasty human flesh a try. At which point he does indeed gain the aforementioned superhuman power and quickly heals up. Ridiculous logic, of course, but within the context of a fantasy-based story, I can accept the supernatural elements.

Really, it's the logical lapses in basic human physiology that I take more issue with.

Either way, it's a fun movie, so I can forgive the flaws.