My son is 3 years old and he woke up this morning throwing up and having constant diarea he wont eat anything yet what can I do to help him?

Focus on fluids. Find an interesting fluid source;electrolyte solution, diluted jello mix(don't refrigerate) etc.& encourage sips all day. More than an ounce at a time may come back up, but work on gradually increasing the volume. As long as he's making pee every 6-8 hrs you have time for his tummy & gut to settle. Applesauce & pasta noodles are good semisolids to try as he improves. If getting sicker see ur dr.
Possible dehydration. Any child with vomiting and/or diarrhea, you get concerned about dehydration. First sign of dehydration is decreased urine ouput. If urine output is still the same, i suggest give small quantities of drinks like pedialytel/popscicles, say start with a teaspoon. If your child tolerates, then increase as tolerated. If urine output has decreased or doesnt tolerate then see your pediatricianl/er.