My Dad (87) is about 5 to 8 lbs underweight. Two Dr's ask that he gain some weight back. How?

If he is physically active and losing weight how about giving him milkshakes as snacks throughout the day?

Define "sick". If you are eating lots of calories and still losing weight, the digestive system is not working properly. He is not well. You and the MD need to have a long heart-to-heart talk.

Since your Dad is ok with digesting milk try Carnation Instant Breakfast... it's a mix you add to a glass of milk. I bet it taste better, and it is a lot cheaper than the already mixed products on the market. The chocolate mix is pretty good, I had to use that after I had lost weight from a surgery.

He actually never has the runs, quite the opposite! I don't like Boost either, but it is easier to get him to drink than eat. Thanks for the suggestions!

Can you father deal with all that dairy, it might give him the runs thus he wouldn't be putting on weight.

Leave out the dairy products except for butter, and try just meat/potatoes, spaghetti, Hawaiian buns with the meal, hearty soups. Ever read the ingredients on Boost or anything similar? Boost ingredients start out saying "water, sugar, corn syrup, milk protein" and other stuff I can't pronounce.