Ok, so you are certain you can trust him around your son, and I understand only too well how difficult it is to date or have a relationship when you already have DC's. As you have said, far more difficult if your child has complex needs.
He chooses to be with you, not anyone else. But as he has autism he probably can't understand your fears and know exactly how to reassure you. If he struggles with showing affection you'll either need to tell him what you need him to do or accept this won't improve.
What can you do to make yourself feel better about yourself? Some people gain self esteem from other aspects of themselves other than their looks. Looks change over a lifetime, we age, we might get ill or develope conditions that effect our appearance. This man says he loves you for you personality and big heart. That's no small thing!
So whilst it would be sensible to try to lose weight for your health, and it would make you feel more attractive, it's the success of doing so, the work involved and the strength of character and commitment needed to do so that should be celebrated. Not how it makes you look.
I guess it would be difficult to go to gym classes or take up running? I always feel better if I can get some exercise, it's not about looking great, it makes you feel great. Could your mum or a friend cope to baby sit in an evening after your son has gone to bed?
Taking up hobbies and challenges is good for raising self esteem. Could you study part time from home?
Do you have a job? Are you a full time carer? If you are at home full time could you research a business idea you could do from home?
In fact, do everything you can to raise your expectations of yourself, set goals, meet them.
To lose weight I imagine you have to alter your relationship with food. Maybe eating is a comfort thing? Maybe it's as a result of not feeling good about yourself? So the starting point is to tackle this from all sides as I've suggested, then look at how you can tackle the relationship with food. You'll be in a stronger position to tackle it. Don't be someone who through lifetime of dieting continues to have a bad relationship with food.
I've never been on a diet. But I've read a few threads on here about the 5:2 diet. I naturally tend to eat this way (mainly I forget to eat!) And I struggle to maintain weight. So I'm inclined to think the 5:2 works. Perhaps join one of the groups on here for support.