I echo Laurie D's suggustion of making a smoothie with soy yogurt. Stoney Field has one ("cream top") that packs 170 calories and 6 grams of protein per container... Use two containers per smoothie and you've got at least 340 calories! You can use rice, soy or coconut milk ice-cream... or non-sweetened almond milk is an option to help make the smoothie liquidy and not overly sweet. Add a banana for an extra 100 calories.
This website offers recipes for soy yogurt smoothies: http://www.stonyfield.com/stonyfield/organic_yogurt/cream...
However, I thought I would also mention...
1.) My daughter is allergic to cows milk protein. Her diet relies on goat milk/yogurt/butter/ cheese. If your FIL is having difficulty with cows milk protein, you can always switch him to goat or sheep diary products. But... if he is actually experiencing lactose intolerance issues (which is totally different from a cows milk protein allergy), then his diet should go towards lactose-free foods--which may still contain cows milk protein. You need to know what his reaction is being triggered by in order to know what foods to switch to.
2.) If his testosterone hormone levels are low, this could cause loss of muscle density... giving him a more skin and bone look (although low testosterone may also result in a pot belly/fat retenion issue). Eating a diet high in soy might not be the best idea if he is experiencing a hormonal imbalance, since soy can be interpreted like the estrogen hormone. So, he might want to get his hormone levels checked to see if that is causing a loss of weight (or a more skeleton-like appearance).