With diarrhea, you really want to keep an eye on hydration - they can go down quick and need an IV. So, keep him drinking. Feed a tsp of canned pumpkin for diarrhea or constipation. Make sure gums are pink (not white). If white, force water immed. and give some Nutrical or karo syrup or honey (to get blood sugar up).
You may also want to give a bit of yogurt (not artifically sweetened kind). GOOD luck!