If I change dad's dinner time to noon, will that help with his lack of appetite? He has been experiencing some sundowning.

Yes. Serve dinner ( supper) between 3:00 and 5:00 p.m. You'll feel better too.

First - check his meds and have a review with dr., if you haven't already consulted him/her on your Dad's sundowning behaviour.
Sure, go ahead and switch the main meal to lunchtime and see if there is any difference. My mother always says that lunch is her favourite meal of the day and that she prefers to have her main meal then, with a snack when she wakes up and a lighter dinner.
Perhaps he is just not hungry enough at dinnertime to have a full meal. Mine says sometimes she doesn't have much appetite at that time a day and is satisfied just having tea and toast.
Doesn't hurt to try changing things around and see how he makes out -- does he get exercise during the day? If so that usually sets them up for a good sleep especially if they have walked outside or been taken outside.
Are you the person who administers his medications? I found my mother getting very confused with hers because there are so many so I took over the job of giving her the right ones at the right time and of course, it made a big difference.
Good luck with your Dad!

Yes, however the nutrition gets in, who cares at that age what time it is?

When my 90+yr young mom started experiencing the same thing, we switched her large meal to the middle of the day. Turns out she wanted another large meal around 5:00, with a snack mid-way thru the 2 meals. That tiny woman can eat like a horse! So I wouldn't get exhausted from all that cooking, after the evening meal I would make up a plate for her to have with the next day's noon meal. Both of us were happy then, and it gave her some independence.

My 91 year old mom stays with me off and on; during her stays she has her main meal mid day because at the end of the day she's not hungry. The added benefit is she sleeps better, too.

Me again. My mother also gets very confused when she is tired. She usually goes to sleep around 7:00. She sleeps a lot more than she used to.

My Mom is 90. She usually eats any where between 4:30 and 6:00. She does most of her eating early in the day. I don't think it matters as long as he's eating and getting enough nutrition.

What have you got to lose? Try it.

Why not just ask him if he's hungry? Their clocks change. I eat when I'm hungry. It's called "grazing." Keep it simple. Whatever works and keeps him feeling calm and secure:) Take care of yourself, xo