After 25 years of marriage, all I can say is that love finds a way. If he truly cares for you, YOUR interests, feelings and health will be top priority. After all, you're only fun and happy when you are safe and healthy. His job is to make sure you are all those things. If he's not living up to that, I'd have to question the strength of the relationship on his side. That's harsh, but reality is best discovered before a solid commitment. And if he becomes committed, he needs to become committed to you in your entirety, with whatever issues you have and those which may arise in the future. To be a team, it takes two who care more for each other than themselves.
I had skeptical family, but they quickly played along when they saw my symptoms improve from being gluten-free. I do not have celiac, but sensitivity to gluten. My oldest son thinks it's in my head and nothing can change his mind. My other son's ex-girlfriend has sensitivity and went gluten free. She's a daughter to a divorced couple and her dad would not put up with her gluten-free needs. He said it was all BS and in her head, so when she spent time at his house, she also spent time in bed and in the bathroom, feeling quite miserable. Those who deny it can have a lot of power over those who do have the issues, and you need your boyfriend to be your teammate, not your challenger. How it works out is up to you, but maybe buying some books or printing articles with things you highlight for him could change his tune. Otherwise, he's probably not the one you want to suffer the rest of your life with if his goal is to selfishly eat whatever he wants and force you to tag along.