I am not a dietician, but a hemo dialysis patient (x5 years), and have had issues with this as well. I have found personally, that my apetite is significantly less then it once was, which makes weigt gain nearly impossible. I completely agree with LeeAnn, that eating often is the key. My mother worries so much about me, thinking that I eat so little at meals, probably about the same as my 5 year old... but the fact is, I eat small portions, but I eat OFTEN. Additionally, ironic as it seems, increasing my fibre intake (while not recommended in large quantities due to phosporus) has really helped ease diarrhea. I try to eat half a bran muffin each morning, and it has helped a lot. Does your dad eat eggs? It would depend on his cholesterol levels, etc... but eggs are a big part of the protein I consume, as meat is hit and miss for me. A meat source that I tolerate very well (might be worth a try), is lean ground beef. If his phosphorus levels are well under control, and he has room to manoevre, yogurt (also considered fluid), and cheese will also increase his protein intake. I have been able to maintain my weight for several years, excluding of course, when the stomach flu comes home from kindergarden!
All the best,