Elderly weight gain

Does anyone have any advice on helping an elderly man gain weight? My grandpa is nearly 84 and has recently loss 10-15 lbs because of his lack of hunger. He was already at a healthy weight and didn't need to lose any, so myself, mother, and grandma are trying to encourage him to eat so that he can keep up his strength and gain some weight. We're trying to find some snacks that are not too filling, so that he'll be hungry for later, but also high in protein for his strength.

So far we have:
Pure Protein bars from Costco
Protein shake with 30 g of protein
Hard boiled eggs
String cheese

Is this a good starting point for snacks? He'll probably have one of these a day in addition to his other meals and snacks. And unfortunately he will not eat peanut butter or any other nut butter spread since he had peanut butter almost everyday as a child. We might be able to get him to try biscoff or nutella, but we want most of his additional calories to nutrient rich, and not just all sugar. He still walks fine with a cane, so we want him to have healthy foods to keep up his strength.

And for those who will ask about what his doctor will say: he's in the process now of changing his primary care doctor now, so once he establishes a relationship with him and fills out the HIPPA forms so that his children can talk to the doctors we'll know more. In the mean time, we're just trying to get him to gain some weight.

Thanks!