Rigtht now do not focus on nutrition -- because the "nutrition packed foods" are mostly ones that will be tough for him to digest or keep down if he has diarrhea. If you give him berries, grapes etc. as someone suggested, that could worsen things -- they are nutritionally great but full of seeds and fiber and skin that are hard to digest!
He will be fine nutritionally and will catch up -- a few days of unsalted saltine crackers (not whole wheat, plain white flour saltines) and white rice and bites of banana will be fine and he will feel better if he is not eating any veggies or fruit or dairy. Ask your pediatrician if you don't believe me!
Please search on "BRAT diet" here on Mamapedia and online; it's for bananas, rice, applesauce (not apples) and toast, and is often recommended by doctors for kids or adults with digestive upsets. Nutritious? Not very, but it gives the child enough to keep going while the system is upset.
And please! No yogurt or cheese or any other dairy. I know the probiotics in yogurt are usually great but dairy is hard on digestion if one is sick. Again, phone your pediatrician and they'll tell you to go very light and very bland. Hydration is key and if the diarrhea is really bad they may ask you to get something beyond water.