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.