My son has been doing it for nearly a year. Absolutely loves it, there is a lot of support, there is a bit of piss taking on the camps but when the adults and older cadet leaders are around there's none of that allowed. Each detached will differ, obviously, but generally any bad behaviour is stamped out pretty quickly and anyone who tried to get cocky is brought in line. In a respectful and sensible way, obviously.
Meetings are twice a week. Start and finish with parade (standing, marching etc). PT (physical training) once a month, not every session. There are fitness tests but general fitness varies hugely and they are really supportive of those who struggle.
Each week has of classroom stuff where they learn theory, history, country code etc. Fieldcraft sessions where they learn stalking and stuff. Lots of tests, both written and practical but they aren't too demanding. They will get encouragement and support if they struggle.
DS has been on 2 residentials and really enjoyed them, though had to leave the last one early as his shelter collapsed under torrential rain and he got soaked through and had a rubbish night, but he was rescued by the older cadets on patrol (DS should have gone for help rather than huddled up and suffered for a few hours!). It hasn't put him off, he learned from it.
I'd recommend giving it a go, at least. It's not expensive and the only kit you have to buy is boots really, they provide the uniform for free. So you won't be spending a fortune then finding out he doesn't like it.