B.C.
Even if he doesn't want to eat, you need to keep his fluids up. Pedialyte and Gatorade will keep his electrolytes up. If he's getting weak, get him to the Dr asap. He might end up in the hospital with an IV to get him hydrated again.
Depending on what is causing the diarrhea, you might want to stay away from (cows) milk and juice. (Breast milk is always fine.) If the cause is bacterial, the sugar in the juice will make it worse. The sweetener in Pedialyte the bacteria can't use. Water is not bad, but it won't keep the electrolytes balanced.