How do I politely tell my boyfriend that he's getting fat?

My boyfriend has put on a good bit these past few months- enough for me to be worried for his health.. I'm also kinda starting to lose physical attraction. How do I speak to him about this?

EDIT: I would NEVER dump my boyfriend just because he gained some weight, I fucking love this guy.. I get why a big majority thinks I'm shallow though- the losing attraction part was only a add on, I care 100x more about his health and honestly I should have only mentioned that. Thanks for all of the great advice in the replies btw.