My prior for this claim being true is ~0. (IE - the evidence that he can go years without food or water should have to be extremely compelling to come close to convincing you.)
What is the number of days a "normal" human can survive without food or water? If I recall correctly it was 3 or 4. Although it may vary depending on the environment and activity conditions.
Well, that's quite a claim. I'd count it more likely that he's a cyborg from the future.
Water + vitamin fasting is sustainable for weeks or months, if I recall correctly. I don't remember the specifics, but I think B12 is the most important.