my old man is very similar... he pretty much lives of meat, potatoes and veg.
he mixes it up, in terms of how he cooks the potatoes, what meat it is and what veg, but basically its the same dinner, over and over and over and over.
very rarely will he have a chinese (and if he does, again, its the same each time, (special fried rice, portion of chips and prawn crackers) ).
the only other things i can think he eats off the top of my head are sandwhiches (usually what ever meat he cooked the night before inbetween 2 slabs of bread lol)
its old people, leave them to it, they enjoy it so its fine usually... if its causing him health issues like you mention, then obviously you need to sort something out though