Defo a torch - a headtorch is very useful so your DS can keep his hands free for other things.
It will be cold at night regardless of day time temperatures so make sure he has plenty of warm clothes for the evenings and a hat in case he is cold in his sleeping bag (keeping your head warm is the best way of staying warm).
If they are going to a site which allows fires, send him with a bag of marshmallows and kebab sticks and he will be very popular (even if the family have them, more are always welcome).
Will he need a camping chair or will the family provide him with one of those? Might be worth asking them if there is any equipment they don't have extras of which you could provide.
Book and a ball and a frisbee always good fun on a campsite.
Check the weather forecast for where he is going - he might need wellies/waterproofs but he should also have suncream and a sunhat - it can be very exposed on a campsite.
Are they going near a beach? If so, obviously he will need swimming stuff etc.
I would have thought £40 for food for the weekend should be fine unless they are planning to eat out in which case you might want to give him some extra spendies so he can contribute.
When I went away with friends' families, my Mum always sent me with a homemade chocolate cake. This always went down very well and might be a good idea if you are a keen baker. (Or a box of brownies is another popular option). If you are nota baker, a bought cake is probably not a bad idea too - always nice to have something sweet for the evenings on camp.