I was told "think high" - high calories, high fat, high protein, high in vitamins. Honestly, around here we don't care if the calories part comes from malts or chips or whatever - all calories are good calories. My former vegetarian and organic only husband has gone no holds barred on food choices. However, in with the ice cream and meats we do make sure he gets plenty of vegetables, etc.
Our challenge is a bit more than some because my husband before ALS was 5.6, 125 pounds and NOT skin and bones. Very low body fat but most of his weight as muscle. Keeping weight on him is essential with NONE to spare, and thus far we have succeeded.
Small changes make a big difference, my husband did not suddenly become a glutton. Butter on the toast, 1.5 times his former servings of proteins, dessert most days, even just little ones. You can so easily add calories to a diet. We have also increased vegi servings.
It's a new thought pattern for a very disciplined eater but it really helped him to realize this was the NEW discipline.