M.O.
Elena's suggestions are great.
I just want to tag on to her by suggesting another way to look at it:
Sooner or later, your son will take a trip without you. How wonderful, fantastic, and amazing is it that his first trip will be with family members, people you really, truly trust? You couldn't ask for a better set of "training wheels."
And since they're family, you have the right to give them all a course in epi-pen 101. Have them run through the drill with the trainer pen until you're confident they've got it down. Have them tell YOU all the symptoms of anaphylaxis until you know they can spot them.
The other lucky thing about this trip is: Rhode Island. Rhode Island is good. I mean, all states are wonderful, etc., but RI is a tiny, densely populated state with a great medical infrastructure. He's not going to be in the wilderness here.
And, speaking of medical infrastructure, first responders are drilled in anaphylaxis and epi-pens like you wouldn't believe. I know, because I am one. [A first responder, I mean, not an epi-pen :)]
So, in the absolute worst-case scenario (which isn't that likely), you're covered. Your son will be okay.
Best wishes,
Mira
(Certified EMT)