For some people, food is like an addiction ( the scientific community haven't come to a definitive conclusion as to whether it is an actual addiction yet.) Just because he seems upbeat and cheery, doesn't mean he isn't addicted.
You seem very concerned that he might not love you as much as he loved his other partners, on the grounds that he stayed thinner for them. But did he have children with them? Having a family changes the whole dynamic. If he felt comfortable with you, maybe he relaxed, the weight went on, and now, like many others, he's discovered just how hard it is to lose. You say he has made some efforts, but gave up. Maybe he found very quickly, that it made him unhappy, so he stopped.
For whatever reason, he currently doesn't feel weight loss is a priority. That seems obvious from what you have written. You can't change him. Nothing you say will help. My husband used to drink. He wanted to drink for all kinds of complex reasons, and if I tried to comment, I was just dismissed, despite the fact I too had some genuine concerns with regard to health and potential impact on children.
I think you have to treat it as you would with any other addiction. Start from the point of accepting you can't change him. Stop focussing on whether he loves you or not. I think that's a red herring. Not only that, it's become a stick to beat yourself with.
Start to focus on you and what you want. Accepting you can't change him, is this a deal breaker for you? If yes, then I think you have to put a plan in place that will allow you to leave.
If not, you have to work out a way you can live with him, as he is, which satisfies you sufficiently. I don't think, for example, you should stop dressing up for him, if that makes you happy. But if it doesn't, then stop. Not as a retaliation, but because you are putting your needs first.
If you don't enjoy sex, then stop. Again, not as retaliation, but because you are putting yourself and your needs first. If you feel that a life without sex is a deal breaker, then you need to think about how to leave.
It may be, that if you change the dynamic (sounds a bit like he is treating you as a nagging mother figure) then he might realise it would be sensible to lose weight, all by himself. Or he might decide your relationship isn't right for him.
Act to make yourself happy. Don't just decide the situation is hopeless and give yourself up to a life of moaning until you are both so unhappy something breaks. You can't change him. Change the way you deal with it.
Good luck OP. Life shouldn't be so damn complicated.