Hi all, OP here - thought I should update. I decided in the end that I was not comfortable with him going. He was bummed (as was DH) but I just couldn't live with it. Planned a fun sleepover with another friend who wasn't allowed to go and was moving on.
Got an email yesterday that so few people signed up that they were canceling the trip and merging with the field trip that was staying more local - Gamboa rainforest for the day, then a camp-out behind the school (5 minutes from our house), then the zoo the next day. Signed him up right away and he's thrilled.
So in the end, I guess I wasn't the only parent who thought it was a little crazy!!

I'm sure he would have been fine, and I'll let him go on whatever trip they plan next year, but I just wasn't ready. I'm glad I'm trusting my instincts - when they offered up the second version, camping out close to home, I immediately felt ok with it in a way I didn't feel about the other trip. So, "crisis" averted and everyone's happy! Rarely do things work out so nicely - I'm savoring it today.
Thanks again everyone for your advice!