You should look for pea protein powder or something like that so you can be sure there's absolutely no dairy in it. Too bad you aren't able to eat much dairy though, as that takes out a lot of easy options to gain weight. A favorite snack of mine that would put the weight on for you fast is Ruffles (potato chips) with cottage cheese. But it's dairy, and quite salty, and I have no idea what kind of phosphorus content it has. 🙁

What about bread though? Or lots of rice? Those are some simple foods that could be easy on your stomach while still adding a decent amount of calories.