Thank you for bringing your question to Just Answer!
As you noted, he is acting like a little child. Since he won't listen to reason, it may be time to stop cooking for him. You can't change another person, you can only change what you yourself do. In this case, the choices seem to be 1. When you cook, separate out his meat and serve it to him plain. Don't bother making sauces for him if he doesn't appreciate it. 2. Cook for yourself, and let him deal with his meal himself.
If he is this way at forty, there's little hope that he will change. This is all about him, and not at all about your cooking, so remind yourself of that when he makes faces picking through food.
Sit him down calmly and explain that since he has such a limited diet, and it's not fair that your diet has to be limited by his, that you will stock burgers & chicken, peas and corn and he can prepare his meals the way he likes them.
It's a little like living with an alcoholic...he won't change until his eating habits cause him health problems (Hitting bottom, so to speak). He might not change even then. But in the meantime, you need to survive and get to enjoy your meals. Forget trying to cook to please him, and just make meals for yourself that are healthy and enjoyable. He'll either learn to cook, or learn to eat what you cook. Either way, give up trying to change him...it's a battle you won't win.