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I live in TX in a rural area and even the small towns around here have senior meal programs. Maybe you can contact the county if your father doesn't live in a large city. These meals are a godsend for elders providing variety, quality and quantity.
There are some very good canned and especially frozen soups these days, and also some nice ones in the refrigerated case. I find myself making a meal of broccoli cheese soup from a restaurant chain and a slice of whole grain bread or an apple frequently. Nothing wrong with a meal of good soup!
Could you bring him a half dozen hard boiled eggs to snack on during the week? My mother really loved it when someone brought her deviled eggs. Simple, inexpensive, and good protein.
Does he take home leftovers from your Sunday dinner? Do you help him stock his fridge with fruit? Can he make himself a sandwich?
It doesn't sound like this is a crisis situation, but maybe there are a few simple ways you can help him add variety to his meals.
(And do look into Meals on Wheels. In addition to the food, it is nice to know that someone is seeing your dad daily.)