Alycia I'm so sorry to hear that your DS has a venom allergy! I myself am allergic to wasps and hornets, and I must (should) carry an epi-pen during warmer weather. There are actually about 8 different types of stinging insect venom, and it's rare for someone to be allergic to all of them. I'm reactive to only two. If you can find out which ones he needs to avoid, then you can learn which ones to look for. I understand your fear of another reaction, but you can learn to deal with this AND let your DS have some fun!
The Epi-pen is one of those things you know you need to have on hand, but you hope you'll never have to use it. It's generally only used in a full-blown anaphalactic reaction, when swelling of the face or tongue or difficulty breathing is noticed. Did the ER staff give you or your son any instruction on how or when to use the epi-pen? If not, try calling back to the ER or calling your son's pediatrician to ask about an instructional video or a consultation. And definitely notify school personnel as soon as possible!!
FWIW, I was stung earlier this summer (in the armpit, as I was picking up my DD!), and did not have my epi-pen with me. Two things that were fortunate for me: 1)I was at a friend's home, and she had benadryl on hand, which I took immediately after the sting, and 2)it was not a species to which I was reactive. Even if it had been an allergenic sting, the benadryl would have helped slow an anaphalactic reaction and given me a bit of time to get to medical attention.