Unfortunately, one reaction is NOT predictive of future reactions.
Yes, it is a beg, borrow or steal situation. I feel like the harbinger of bad news, I am so sorry I know it's a tough situation. On a positive note thus far shrimp alone should be easier to avoid than peanut or milk for child care purposes.
An act allowing schools voluntary stock epinephrine was signed into law. I wonder if it's possible for your doctor to write a generic prescription for your daycare center and the preschool would agree to it.
Emergency action plans typically call for the administration of Benadryl with one symptom, epinephrine with two or more. I'm hesitant to say when is the time to administer when I wasn't there. It's said many cases of anaphylaxis self-resolve but the kicker there is it's a complete unknown. What we do know is the rate of mortality decreases with prompt administration of epinephrine once you know you're at that moment. Under "Main" section are a series of stickied posts. I'd start there and be warned, the learning curve is steep. Like any serious chronic condition that is life-threatening and episodic you'll learn the ropes and cope.
I'm trying to not overwhelm you with information. The big ticket item is some sort of epinephrine autoinjector that goes with child EVERYWHERE and those who care for child know how and when to inject then call for rescue squad. I'm afraid the only preventive medicine is low tech avoidance of the trigger. How much of the allergen your child can tolerate before reacting you won't be able to pinpoint.
1. Beg, borrow or steal to get the EpiPen, Jr. prescription filled. I'd go straight to county health or try CHIP or ACA. I'd even ask the doctor's office if they have some sort of legit 'sample' EpiPen, Jr. Maybe their pharm rep can cough one up. There's no substitute for epinephrine in treating anaphylaxis.
2. Safeguard that in a medicine bag labeled for child. It needs to go with child always. Keep it stored according to manufacture specifications.
3. Take a look at EpiPen website for injection tutorials, recognition of anaphylaxies. FARE is an organization that should have this info as well. For that matter I'd call FARE tomorrow to see if they can help with the Epi prescription because the makers are hugely tied to FARE.
4. Pass everything along to anyone caring for your child. Not only treatment in emergencies but in avoidance. What he is served to eat.
5. Check in "Main" on this site for the anaphylaxis grading chart and maybe FARE or Mylan (EpiPen) website for sample action plans.
Hopefully someone here at FAS eyeballs what I wrote to factcheck me for errors, improvements.
If you feel comfortable revealing your state some of us could probably help with finding a good allergist and maybe resources to get your son that EpiPen, Jr.