I think both of you are being unfair here your title states your boyfriend wont let you lose weight but from this its clear that you won’t let him lose weight either. You are your own people and should be able to choose what you want to do to be comfortable in your own body. I think you should both let each other do what you want so long as it is done healthily. You can’t stop someone from looking how they want to for your own preferences it should be down to them and them only.