Hey Tracey, looks like you're pulling up a beach chair and sticking your toes in the sand! Welcome!
I understand your concerns about feeling like you'd be 'outdoing' your boyfriend who just won't be able to do as much as you can because of his illness. But, by getting your health in order you are really doing him a favor - what would he do if you had a health crisis, which is more likely while you're carrying that extra weight? Also, perhaps your example will encourage him to try to find other ways to move his body than the bike machine you talked about. I'm a big believer in heading off problems at the pass, so if I were in a similar situation, I would sit down with him and let him know ahead that not only are you committed to getting your body into the best shape it can be but that you are concerned that you would be hurting his feelings in the process, and try to get him to talk to you about it. You may be pleasantly surprised at his reaction - my DH was so relieved that I would talk to him about my expanding girth because he was desperately worried about me but thought he would hurt my feelings if he mentioned it. The nice result is that he's an active cheerleader in my household and he has been really supportive about encouraging me when I'm down, and gently reminding me when I've skipped my exercise.
A word about exercise, also. I like Leslie Sansome's Walk Away The Pounds DVD series. They are designed for those who may not have a high fitness level to begin with, or who don't want to learn complicated steps or moves to exercise. Her 1-mile walks are about 15 minutes of light aerobics, and you are encouraged to go at your own pace. I got my mom, who was extremely ill for a time and had almost no muscle tone afterwards, a 1-mile/2-mile combo DVD, and she did the 1 mile routine most days for about three months before she even looked at the 2-mile. But she saw improvement as she went along, and now she just did the 2-mile for the first time and is really excited that she wasn't too exhausted afterwards.
And I hope you've checked with your doctor about your plans - I started my diet and exercise program before I checked with her and she got mad at me! (I talked her around once she heard I was doing SBD)
And looking back at your original posting - it would be a good idea to either have your boyfriend talk to, or you both talk to his doctors, he must have a specialist for the muscular dystrophy, right? Let that doc know what you plan, and ask him about possible ways to get your SO more acitivity. This diet will work if you don't exercise, but exercise is a natural antidepressant, and it may help your SO to feel stronger in himself even though he is struggling with difficult limitations.
Best of Luck! Let us know how it's going, especially if you're having trouble. That's what has been so great about this forum for me - when I'm freaking out, or totally off plan, or depressed or whatever, I can come on here and the ladies give me a gentle shake and group hug and suddenly I can at least think straight about what went wrong and what the next step is. (thank you again, Lovely Ones!)