Bf thinks he’s “fat”

My boyfriend is 28 and seems to have always dealt with body image issues. I’ve been with him for a little over a year and over the past few months he has put on a few pounds. He has been dealing with some recent trauma so it’s to be expected that he’s not eating the best. I honestly didn’t notice until he started talking about it. I guess I see it when he wears a tight shirt, his stomach kinda pops out a bit but when he’s shirtless I honestly don’t see it. He’s six foot large chested so the weight doesn’t look bad on him. But he keeps making the “I’m fat” comments and I always tell him he’s not or say I still think he’s sexy even if he thinks he’s bigger. I’m not really sure if there’s a better path for handling this. He overall tends to think he’s unattractive so gaining weight for him is a bigger deal. He also recently transferred jobs so he no longer does any manual labor that he would claim was his workouts. I don’t live with him so I can’t quite force better eating habits on him and i don’t really have good eating habits myself, I eat chips for most of my meals or accidentally forget to eat for a day or two due to high stress work but I never really gain weight. Is there a way to help him deal with it? Or comfort him?