I suggest doing a search on "kidney diet" or "low protein kidney diet" on the web.
In general, most of the protein in our diets comes from meat, dairy products, soy and other beans.
He can aim for less protein (especially less animal protein, which is harder on the body than plant-based protein because it contains more sulpher-related amino acids) by going towards plant-based diet, and avoiding too much soy, beans and other high-protein plant foods (seitan, for example).
Rather than pastas and potatoes and other "white" carbs, stear him to healthier, veggie-based carbs (broccoli, green beans, red peppers, foods with lots of color).
If he's anything like my father was, this won't be easy! My dad was a real "meat and potatoes" eater (and died of colon cancer, which is tied to high consumption of animal foods). Though he did like veggies from our garden.
I'm not sure how "low" the doctor wants him to go. Did he give a %? Usually, less than 10% protein wouldn't be enough for health and maintenance, even for an inactive individual. Try to get a more specific guideline from the doctor about % of protein. Then you can get a book like "Composition and facts about food" (or something like that), that will tell you the breakdown of various foods in carbs/fat/protein. I've seen some books like this in the health/diet section of local bookstores.
Actually, if you get an Atkiins book from the library, see what they recommend, and see if you can do the opposite. (ie: they will say that tuna or X or Y or Z is a good source of protein, you'll know to stay away from it, or limit it).
