Well, as far as the food goes, I disagree with most.
We cook one, and only one meal each evening for the family, and it is a meal that is suitable for everyone's dietary needs. HIs need to lose weight is a medical need, and I think that should be accommodated just as an allergy or intolerance would be. If he needed to eat a gluten free diet for a genuine medical reason, would you really refuse to accommodate this in the family meals, and expect him too cook separately for himself?
Outside of the main meals being suitable for all, there's no need for anyone else to follow his diet, although I'd think it unkind to be scoffing junk food in front of him or particularly rubbing his nose in it.
That said, as someone else pointed out, this isn't a food issue, its a far deeper relationship issue. Assuming you both work equally, then you should be sharing cooking and meal prep. You shouldn't feel like you are accommodating him- you should feel like a family who all look out for each other, and it seems that there are deep rooted issues in your relationship.
But, to answer your question on face value, I do think that family meals should be cooked to accommodate every family member's medical dietary needs.