My three-year-old got a wasp sting. Is there a remedy?

Wasp stings are very stressful at any age, especially because the wasp will keep on stinging if it is able, unlike the bee which can only sting the once. I will assume that your son is not allergic to bees or wasps, since you haven’t mentioned anaphylaxis in your question — as this is an entirely different situation needing medical attention.