My 4 year old son vomits, have a fewer and diarrhoea. he only wants water water and then vomits. what can i do?

Watch the volume: The kid is smart to want water, but sometimes the volume can bring on more vomiting. This sounds like a typical gut virus & often the stomach spasms when they drink, triggering vomiting. Cutting the volume down to an ounce or two at a time, but every 5 min or so can help him hydrate but avoid vomiting. The volume might be increased as tolerated/reduced when not.