I definitely wouldn't nag him about it. I know that if my husband had started nagging me about my weight and what I eat I would have been seriously hurt and it would have damaged both me and our relationship. Weight loss is a pretty personal journey and until someone really wants to make a change and consciously chooses it for themselves, nagging just won't work and could backfire.
Hopefully he'll eventually be inspired by your success and your happiness in making healthy choices. I know I was inspired by my mother (who NEVER, EVER nagged me or said an unkind thing about my weight) when she made lifestyle changes and lost 30 pounds...
I can't relate in the sense of worrying about the SO's health and weight, because my bf is an ectomorph, through and through! I won't say he eats horrible, but probably about twice as much as I do. He gets to indulge in bad treats whenever he feels like it, too.
I wish I had an answer. It's hard to watch someone eat a juicy cheeseburger while you munch on lettuce. It's even more frustrating being the fat one! My best suggestion is to ask that he not do it in front of you. I agree with the others that nagging is going to exacerbate the problem.
Thanks everyone. I talked to him and told him I wouldn't support his habit I wouldn't comment on it. I told him I would love for him to join me at the gym, and offered to pack him healthy snacks for work. I'm just going to wait and see, and take care of me right now!