What meals can you buy at the store for an elder dad who lives on their own and is losing weight?

Carbohydrates, carbs, and more carbs. Buy Pasta's, or make it yourself in a big pot. You can portion it yourself in small meals for him. Make garlic bread and a nice salad with it. Tuna caseroles, turkey meatloafs are also good. But pasta's are by far the best.

You can also try meal replacement drinks like Boost or Ensure, and use them as a regular drink with his meal. Otherwise he can try healthy carbohydrates like whole grain (brown rice, whole grain bread, etc.) and maybe try the Hungry Man meals.

Here in Oregon we have Loaves & Fishes (Meals on Wheels) they will deliver food to the elderly everyday if necessary( except holidays) in which case they deliver double the day before. It costs either nothing, or VERY little.