What is eating you? = What is eating him? = What is eating her?
what is worrying or annoying you (or him or her)? - informal
RELATED IDIOMS :
have someone eating out of your hand
have someone completely under your control
1987 - Bernard Mac Laverty - The Great Profundo - One of my main difficulties is that I'm not good with an audience. There's guys can come out and have a crowd eating out of their hand right away with a few jokes.