For people who aren't insulin dependent you can manage on carb free food, but if my son's roaming about the woods and doing climbing and abseiling, he's going to need some carbs!
I think that's the wrong approach.
I realise there's various practicalities involved but what you're really describing here is feeding the insulin. The reason people without diabetes don't need carbs to carry out activities without risking low blood sugar is because their pancreases stop releasing as much insulin. So the same logic applies here - if your son is going to be climbing, abseiling etc., rather than 'adding in' carbs to stop his insulin sending his blood sugar low, stop 'adding in' the insulin. If his basal rate is reduced AND he is exercising, providing he gets the right balance, there's no need for extra starchy carbs to stave off hypos.
I know, it's a lot easier in theory, than practice! But this is a really good habit to get into. I think as a whole, everyone with diabetes, when they're having trouble with low blood sugar, always defaults to adjusting their diet first, rather than considering insulin.
My rules are generally if your problem is low blood sugar, it's your insulin you need to change, and if your problem is high blood sugar, it's your good you need to change...and possibly your insulin too, but food first.
I think with the 'wild cooking', it's perfectly fair for your son to carry glucose tablets and I bet they're not so rigorous as to prevent the kids from bringing drinks with them, so a Coke or Lucozade in a backpack should probably be fine and then at least there's a corrective treatment for low blood sugar available. If they're making twists (that's what I called Copepod's damper mix when I was a cub scout!), they'll probably have jam as well.
I guess really though, it's about knowing what to expect. Have you asked the school what exactly is meant by 'wild cooking'? It might be they know exactly what food is going to be available on the day (I can't imagine they'd take a bunch of kids out in the wood without at least some sort of guaranteed food plan), in which case you can build a plan based around that.