Severely obese spouse

My husband is severely obese.. his current weight is 480pounds. I am not tiny I weigh about 190 and I have major back problems/pain. I had a rod placed in my back in 2011 and cannot bend much during sex. The last couple times that we have had sex I have had to fake a cramp in my leg to get him off me so I can breathe without hurting his feelings. I know the easiest option would be for me to get on top but we have tried and failed with that! He is so wide and my back is so stiff it just can't work. The only comfortable way for me is doggie style but he doesn't want to do that every time. I honestly don't know what to do! We want to get pregnant again but I am at the point that I hate having sex with him because I physically hurt every single time we have sex! I am open and honest with him and tell him that he needs to lose weight but that will not be a quick fix. I want another baby so bad but just don't know how to have enjoyable sex!!! Any ideas or encouragement?????