Was he like this since you met him? If so, you should leave him rather than try to make him change his ways. This is who he is. You either accept it or you don't.
If you choose to neglect this advice... my partner has a history with eating disorders. When I notice her backsliding I remind her to get something proper to eat before work/gym/drinking alcohol. However, the difference here is she has asked me to call her out if she skips too many meals.