I found out last summer that my 4 year old daughter was very allergic to fire ants when she was bitten 20 times and had similar symptoms to your son. Yes, it is very frightening to see your child suffer from anaphylaxis. It's something I never want to experience again. Unfortunately, living in Texas, fire ants are everywhere and it is impossible to avoid them completely. I highly recommend that you see an allergist. We see Dr. Santilli at AIR Care. I can forward the information to you if you want it.
My daughter goes once a week to get an allergy shot to "immunize" her against the fire ant venom. A few months into getting the weekly shots, she was bitten once on the toe and had only a local reaction. This was a HUGE relief for me, but we still have a long road ahead of us. Allergy shots are recommended for three to five years, but it is well worth it in the long run. Once she has completed her treatments, she will have the same immunity as someone who is not allergic.
She keeps an epi-pen at school and will always need to carry one with her wherever she goes until she is immune. I make sure all of her teachers/caregivers know how to administer it. It comes with a DVD and a practice pen. I insisted everyone watch the DVD and practice with the pen. It's very easy to do, but in an emergency situation, people tend to panic and it's important to prepare for these events.
Anyway, I found out from the allergist, that fire ant allergies are fairly common. Also, if someone is allergic to one kind of insect bite/sting, they are often allergic to others. I had my daughter tested, but she is not allergic to any others such as bees, wasps, or hornets.
That's my two cents. Just know that you are not alone in your fears and concerns, and that things can get better with the proper treatments.