My father in-law used to love steak. But now he doesn't know what he likes. If you ask him if he wants steak, he says no I don't eat that stuff. And that's his answer for every other food too. If you ask him what he wants, he says something edible. Should I just cook whatever and put it in front of him? I understand people with dementia can loose the ability to remember what things are called so maybe he doesn't know what steak is anymore.
Just cook whatever and put it in front of him. If he likes it, great, offer it again soon. If he hates it, don't put it in front of him again any time soon.
Certain kinds of dementia effect the sense of smell. (My husband lost his.) That in turn makes food taste different.
Some kinds of dementia (most of them, eventually) make swallowing more difficult. Someone who used to love steak may not feel safe eating it now.
I'm afraid you are just going to have to rely on your own observations about what, if anything, he seems to enjoy eating.