Pinworms are so highly contagious, that reputable doctors treat the whole family whether they are showing symptoms or not. Not everyone shows symptoms, so your whole family probably does have pinworms already. Your son is probably getting reinfected from things within the house. It takes 1-2 months for eggs to turn into worms. Your doctor gave you 2 pills because you're supposed to treat pinworms twice, 2 weeks apart. If your doctor didn't explain this to you, get a new doctor and make sure you are fully aware of how to use any medications he prescribes. Your son (and everyone else in your family) need to get on medication IMMEDIATELY if you want to get rid of these parasites. You need to wash everyone's bedding daily, bathe everyone daily and keep everyone's hands clean, no matter how hard this is. Pinworms, like many child-related diseases, are something that happen, but you need to gather knowledge about them as you encounter them from medical professionals
(try www.webmd.com for advice/information instead of mom's sites). I've given you all this advise, but you don't know me from Adam and I'm not a doctor, so PLEASE get in to your doctor's office ASAP, for the sake of your son!