You can ONLY give him congee (i.e. rice porridge) and some water. Not thick congee, don't make it like rice, more like liquid, don't add meat or vegetable.
You must let your baby have it until one more day after your baby doesn't have any more diarrhea. Don't give your baby milk or any other solid. Let your baby's intestine take some rest to recover for itself. Don't worry about nutrition. A baby can survive very well with just thin congee, for a week or so, or even longer. They will return to their usual size again very easily after they have recovered from sickness.
Apart from that, you can also observe if your baby is sensitive to some flu medicine like antibiotics.