if the mass of the moon is 73,476,000,000,000,000,000,000 kg and you use E=mc^2 to turn mass into energy you would get 6603693551286641291664000000000000 MJ and turn that to calories you would get.
1,578,320,638,452,830,190,000,000,000,000,000,000 calories
In conclusion, the old man ate a lot.
(edit)
For him to use up all that energy it would take up to 2,702,630,700,000,000,000,000,000,000,000 years.