Not Allowed to Gain Weight

Okay, so this has been really eating me alive for a long time now. My bf and I have been together for almost two years, live together, and have a cat together. I love him so much and we get each other in every way. Emotionally, romantically, and sexually we are so compatible and well suited for each other. Here’s the problem. I have an eating disorder that my boyfriend is aware of. I’ve been struggling with anorexia since I was in high school and have been in therapy for this and some other things for years. The problem is that my bf loves how skinny I am. It’s one of his favorite things about me. He grabs my waist and compliments how small it is, talks about how petite I am, and generally obsesses over me being skinny. And now that I’m working on myself in therapy I’ve gained a little weight and am just crumbling. I don’t want him to see my body and just feel devastated about my looks. I’ve asked him blatantly if he would leave me if I got fat and he has always said yes, he would leave me. I ask him if he would still be attracted to me if I gained weight and he avoids the question or eventually says yes after I point out he is refusing to say yes. I’m just so torn and so uncomfortable with my body. I know what I would tell any girl in my situation- leave him. But it isn’t that simple. He’s everything to me and we have such an amazing relationship. It’s just this one thing. What do I do?

For reference I was a size two when we met and am closer to a four at the moment.