My H had an awful diet when we met but we were younger then.
He then started coming round and I had made a bit extra so he started eating what I cooked.
His skin, energy improved.
He always paid for every drink, nearly every drink, every meal out and take away. He always brought beer/wine/anything we messaged him from the shop so it's not like he was taking the piss.
He just didn't see making food as part of being social. But that grew together.
So perhaps you could put Dom's music on at the weekend, marinate some chicken, make X nice potato salad, green salad, get good bread in and have cooking together as part of the weekend?
Some people just have never been around a home based around the kitchen.
I know so people think "oh if he can't cook then I couldn't be with him" but that's too black and white thinking for me.
Me & H still together, 20 years, I still make most of the food, he still pays for everything. I make us, and our now 2 teenagers, a roast most cooler months Sundays, I relax, have a glass of wine, radio podcasts on, we eat, talk, play a board game. The kids cook now, radio on we all chat, it's social and you end up with home cooked food and having spent fun relaxing time together.
That could be what is missing from his knowledge.
Might not be of course.
But just another point of view.