How do I make him listen to me?

My boyfriend of over a year eats junk day in day out, he hardly ever eats a proper meal and just lives off basically snacking- biscuits, chocolate, crisps and fizzy pop. He's actually underweight for his height but I know it's affecting him cause he gets really moody sometimes and as well there's the whole thing with it affecting his future health. He says he 'hates' healthy food, so how can I get him to do something as simple as eat a bowl of cornflakes at breakfast or spaghetti bolognese at dinner?