How can I help? Is it my fault?

my boyfriend gained an unhealthy amount of weight. He always says that he will start diet next week. And he actually did it for a week and now he is back to eating fast food with colleagues once a week, snacks during the day and other unhealthy meals. We don't live together, but all the time when I'm with him, he says he wants to relax and have a cheat day. Work has been stressing him out in the past. I try to convince him to eat vegetables or fruits. Anything healthy, anything that would help his diet. But now even when we are going to the store he is buying snacks and other things. I like being around him, but maybe I shouldn't so he can continue his diet. I feel guilty, cause I can't help him. I love him no matter how much weight he gains, he just tells me how much he hates "being fat".

Small note: We were separated for some time, because of him. Talking about his weight makes him uncomfortable. He said he was feeling depressed without me, but wanted to have me back so he did everything to lose weight. Now his weight is just getting worse again.