Go into his bedroom at night when he is sleeping with a flashlight, pull down his underwear and shine the light on his anus, if there are pinworms they come out at night to lay eggs. Use a piece of kleenex to grab them. You can also put vaseline around his anus at bedtime and the worms get stuck in it. You need to make sure he wears underwear, keep his fingers out of his itchy butt and out of his mouth (reinfection), wash hands frequently, vacuum his bedroom daily.You need to wash his underwear, bedsheets and pajamas in really hot water Kids usually get pinworms from other children or cats (litter boxes, sandboxes that cats have used as a litter box). If he plays with other children who are infected it is likely he will get reinfected and you and your family can get them too. I used to recommend Vermox (mebendazole), it was an easy to use, low toxicity pharmaceutical, one dose and you were usually done. It seems they stopped making it but no one I have spoken to knows why (it was not due to lack of efficacy or due to adverse effects). Another company now makes Emverm which is the same medicine at a really high price $702.00 for 1 pill, which is usually all you need, stills that is highway robbery.
I usually treat for a month solid, and the herbs I use are gnarly and not herbs I like using for 3 year olds - Neem, Quassia (has toxicity for small children), Artemisa annua, Elecampane, Pippali Long Pepper, Black Walnut, Wormwood (not great for small children either) and garlic. Dosage will depend on the formula and use his weight divided by 150 to determine accurate dosage.