What can i do to help my 3 year old stop vomiting?

Let his stomach rest: Clean him up and comfort him. Do not offer anything for the next hour or so. If he is comfortable, allow him to rest. This rest will allow his stomach to settle. Later offer a sliver of an ice pop or ice chip to suck on. If he tolerates it , allow a bit more after 10-15 minutes. If he is not vomiting-increase slowly the amount. Sips from a spoon can also be utilized. If persisits get checked.