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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!