I agree with Mary.  Give him time.  Find ways to empower him. 

Given his age, things we've done with our sons is to have set rules about eating things outside of the house.  It gives them clear steps to follow and a bit of knowledge to make them feel a bit more in control of their food allergies.

Some rules we have:

Don't rely on someone else to tell you if a food is safe or not. 
If you can't read the ingredients on a package then don't eat it.
Don't eat things baked/made from someone else's house.

It is possible when you sense that your DS can talk about what happened, that you can reflect over the situation and think together about what to do differently in the future so that something like that doesn't happen again. 

So sorry your son is struggling.  It takes time after and Epi/ER visit.  Give him time.  Have him read labels to empower him a bit maybe? 

DS 11 had a bad reaction after eating a cookie at day camp this summer.  (He asked the boy who offered it if it had nuts; the reply was no; actually it had pecans.)  We used the epi-pen and took him to the ER.  He was fine, but now is nervous around desserts in general.  He came back from two successful weeks and told me that he no longer wants to eat chocolate because he feels like he is going to have a reaction, even though he knows he is not.

I feel sad that he is giving up an additional food that he used to love, but I want to be respectful of his boundaries.  Any advice from BTDT folks on how to manage the after-effects of a reaction?