Late stage dementia and food/drink

Always offer food and drink.
You may have to help him.
But if he keeps his mouth closed or turns his head then stop offering at that time. you can try later or with a different food or drink.
To get someone that still wants to eat to open their mouth if you place the tip or edge of the spoon just under the lower lip and apply a little pressure so they know the spoon is there that is sometimes a cue to open the mouth.
Pudding, Yogurt, Rice Pudding, apple sauce, grits, scrambled eggs are all things that are easy to eat.
You may want to avoid things that are "slippery" like peaches, banana, nectarine as they can slide down the airway. These items would be fine mashed so that they can be eaten with a spoon.

I would continue to bring him very small portions of some of the things you know he likes and then leave it up to him to decide whether or not he wants a bite or two... if you sit down to dine with him he may feel more like eating, plus sometimes the sight and aroma may whet an appetite for a taste of something. But this should be all about offering something that may give enjoyment, not coercing or guilt tripping him into eating.

When a person is dying their digestive system is the first to start shutting down thus why folks will not eat or drink, as it can be quite painful if they were to try.
So you never want to force either food or drink on a dying person.
If your dad was hungry or thirsty he'd welcome your offers of either food or drink, so the fact that he doesn't want either tells you that his body is shutting down and getting ready for the transition from this life to the next.

With his Dementia, he probably has no idea if he is hungry or not. No is their default word. Place some snacks near him. Get some ensure. Get is nice and cold, and hand it to him. Don't ask if he wants it, the answer will probably be no. Just hand it to him or leave on the table near him. If he seems to have problems swallowing and starts coughing after drinking and eating, then he maybe aspirating food. It could be his body shutting down, it could be age and Dementia. This is where you call the Hospice Nurse.

Offer and then accept his answer. My dad didn’t eat at all for five days before he died, and his appetite had decreased a lot before that. Our wise hospice nurse suggested I make ice chips from my dad’s favorite drink, so he’d get some hydration and still have the flavor he liked. This proved a great suggestion. He took the chips off a spoon regularly like a baby bird until a few hours before he died. I wish you peace during this tough time.

Keep a Glass Of water around and Moisten His Lips with a wet cotton Ball . Don't worry about him eating or drinking his Body is shutting down . My Mother had No appetite at the end .