My 2yr old son has had a tummy ache for 3 days now. No fever. He did vomit yesterday, once. Low appetite and clingy. Best ways to ease his pain?

Many possibilities. Abdominal pain for a 3 year old has many causes, starting with a viral or bacterial infection of stomach, and intestines, constipation, uriany tract infection, food poisoning, lead poisoning, allergies to name a few. Also it could be stress-related. If it does not resolve on its own in a few days, i would suggest that he needs to be seen by his pediatrician.