My grandparent's cat eats way too much and is still skinny why?

My grandparents cat is now 14 years old, he is always hungry and often steals food from the other cat who is only a few years younger. Both are males and at first i thought it was a dominance thing but now i'm not so sure. Sammy (the 14 yr old) is so skinny that you can feel the vertebrae in his spine. And he's so hungry all the time and always begging. I don't know what to do with him. And i don't know what's wrong with him.