Some of the more active churches might have summer activities, there are also various Christian summer camps where they go away for a week somewhere, if that's at all relevant to you.
Got nothing against that, but I know a lot of them require you to be Christian (IIRC my mum managed to blag me into one of them when I was younger but I had to pretend to go to church etc.

+1 for joining cadets, get to do lots of interesting stuff and learn all sorts of skills. I was in the army cadet side of a combined cadet force when I was at school and would definitely recommend, but go for the air cadets if he can. Might have changed, but the unit I was in the air cadets got to fly gliders & planes AND do most of the stuff that the army cadets do.
Maybe check out local sports clubs that might have some summer activities planned?
I remember there used to be some academically focused summer residential schemes that some people went on, I never found out much about them though so no idea how they were organised. They're probably more common for GCSE or 6th form aged kids, but various organisations ran academic residential courses.
Also some universities have short summer residentials, some of them might be limited based on your demographics though.
Thanks, I'll take a look at those and see if there's anything suitable.
Not sure what else tbh though. Imagine liability and safeguarding issues will have put an end to lots of the more informally organised activities from years ago.
Unfortunately I think this is probably the case, but even more formal groups seem to have very little on - been looking on the RYA site for sailing/windsurfing courses, and while there are lots of links to sailing clubs on there, I go to the site and they're not running any courses, or they're only taking bookings for 2023/2024

Air Cadet camp saw me spend a whole week working in the armoury, servicing both small arms and aircraft weaponry. Another camp was doing ground defence training with the RAF Regiment, which included NBC training, some time on the CQB range, a few days doing some parachute training and a day playing with RAF Police dogs. Then there were all the other camping and outdoor activity things like windsurfing, raft building, hiking, navigation, rock climbing & abseiling, bushcraft, fieldcraft, etc etc... and if you had time, a flight or two in something or other. Chipmunks and vigilant gliders were at some stations, but active stations had fun things like a Chinook or a Hercules.
Subs cost 30p a week back then, but IIRC camps were completely free, with food and accommodation all provided. The only money involved was what you took up the NAAFI in the evening.
I did ATC for a bit when I was 15-16, and seem to remember it being along the lines of 70% being yelled at, 20% RAF recruitment and 10% fun activities, yeah we did some time on the range with the L98s, flew a Bulldog with a 90 year old WW2 vet and some fun navigation exercises and needing to "raid" an airfield in the middle of the night while the officers posed as guards, but the vast majority of it was drills and marching.
One of his mates does do ATC, and we also have a Sea Cadets group very nearby, but I'm not sure of the merits of joining just over the summer, and he has enough on during the week in term time that I think joining full time would be a bit too much for him (plus the ATC is on the same night as his Scouts group)