As the others have said, the doctor can prescribe FORTISIP, a liquid in cartons, which contains every nutrient a person needs - 3 or 4 cartons daily usually.
The operation you mentioned wouldn't be offered to your dad, because of his age anyway.
The oesophagus narrows sometimes in elderly people. Your dad could try having rice puddings, blancmange, custard, etc., all made with full-fat milk.
Another Tired Kicking For The Sick And Disabled By Rishi