I have been in a relationship for 9 years, with a man who once cooked pasta by boiling it in a kettle...
Our first argument was about toast - he (sheepishly) rejected a sandwich I'd made for him as if it was toasted it would have 'less calories'. He must have seen my lip curl, and it was all FINE FORGET IT.
He was a fat kid, who lost significant amount of weight as a young man. He has strange beliefs about food including 'good' and 'bad' foods that will make him put on weight overnight (ummm that's water). I was a skinny kid, who has to work fairly hard to be overweight. I love to cook, I'm the main food prep in our relationship, and I'm good at it.
The biggest challenge was when we started living together, to be honest. In one hand I'd have him overeating and then saying he felt sick, in the next he'd be saying he was fat and ugly. He's a naturally big human being, and mostly I've always found him attractive so it pained me to hear him say things about himself. It was extremely difficult not to get into a cycle of policing his food choices. Or to cook him a low carb thing to be supportive, only to find he'd eaten a pizza for lunch...
For the past 6 months, we've been following a slow carb, low sugar eating plan together and it's been fairly revelatory. We're finally more equal in our food relationship, I've worked on not commenting/ policing his decisions. We plan weekly menus together. He's an adult, he gets to decide what his does with body. He's also lost 23lb, where I have lost 5 :)
I'd caution you to go into this a) expecting change b) that you can do anything to influence change. And issues of weight are much, much more deeply embedded than you realise. I always thought I had a fairly body positive/ food positive outlook, but I was completely stumped by our dynamic together. Two books that helped us were the Beck Diet Solution (CBT approaches to dieting) and Always Hungry (the slow carb eating plan). However, I don't think we were ready for that until we'd lived with each other for 6 years...