Footwear - both. Wellies are great for first thing in the morning if you have to trek across to the toilets - easy to just jump into without faffing with laces (do make sure they are big enough though) but they can be a bit clumsy for some activities. Walking boot type things are best, but don't go buying them - ask the group if they keep a stock you can borrow or buy 2nd hand off them (a lot of Scout groups do this for example - don't know who your dc is going with) of try asking on Freegle, or around friends to see if anyone has any you can borrow. Some dcs wear wellies full time though, so not to worry if you can't.
Sleeping bag - I wouldn't buy one where feet touch the bottom. 1) you need to be able to snubble down in it and scrunch it up round your shoulders and 2) if you are paying out good money, you want it to last several years, and he's only going to grow. There is unlikely to be room for a duvet, but a good investment is a roll mat. Honestly, it sounds illogical, but the more layers you can get between you and the ground, the warmer he will be. Also, make sure he has a warm hat and knows he can wear it at night (if he's got more than one, pack one in with his PJs)
Snacks - I'd say no, but this varies group to group so ask the LEaders.
Torch - Either use a wind up one, or put the batteries in upside down AND SHOW HIM HOW TO PUT THEM BACK so it doesn't get switched on in the bag and end up with no battery when he wants it.
LAbel everything clearly - several children sharing a tent means clothing WILL get lost and if it's clearly named you stand a chance of other parents returning it to you
Send a good waterproof with a hood, and sun cream! British weather is unprdictable.
Make him pack his own bag, or at very least, pack it together, so he knows what/ where everything is.