If something as weak as weight gain can make this person leave you, he is NOT worth having in your life. What if your physical appearance was effected by something that couldn't be reversed or controlled? Excuse my bluntness sweetheart, but the love in your relationship sounds one-sided.
He is as shallow as an adolescent boy, and I really don't see the point in you sticking around. There are plenty of men that are only attracted to slim women, but the love he was SUPPOSED to have developed for you over the five years you two have been together, is SUPPOSED to help him see past that. The way I see it is if you put down a fantastic book because the front cover got ripped and torn, there is no way you could've been enjoying the story. And frankly, he certainly doesn't deserve to be kept around for the happy ending.