For me personally I have always taken them after 24-48 hours of diarrhoea. If I see any signs of dehydration, lethargy or fever I usually take them straight away.

Make sure that the food he gets is easily digested. To help him overcome it give him binding foods like boiled rice and bananas and reduce fibrous fruits and milk products. If he is on bottle feeding consider replacing with gastrolyte or something to help hydrate him. If he is breast feeding, feed him as much as you can.