I think it’s ok the cause of you not being able to sleep, you’re worried about something where you don’t know enough about the circumstances he’s in. My mind does the same, tries to fill in the blanks (except I end up worrying and thinking of worst case scenario).
Basically unless he’s in a gang, gangs don’t really affect you. They must exist but I’ve never seen any and I spend half my time here. The point of gangs is to sell drugs and horribly get schoolkid boys (usually) to act as go-betweens. It’s awful and no one cares, you never see police raid a bar to catch the rather snooty users. So if your son isn’t in a gang (he won’t be) then he’ll be fine. He might need to watch for pickpockets on the tube and maybe standard drunk people - I mean it’s possible but super low risk, will be the same as your area.
Hope that helps! The most important thing is… he enjoys his time here :) think of all the up-sides… making new friends, having fun in his job and maybe meeting a partner. All the great food! Galleries and sports stuff on all the time :)
What YOU can do is make sure he feels super supported. When kids move away some parents are honestly crap - I remember in my first houseshare, one girl’s parents never called (for two years). He didn’t go home for Christmas. They hadn’t had a falling out, they basically said “well you’re an adult now so off you hop”. It was awful, she felt really disconnected from them and it made her question her childhood. Others had parents who moved abroad and left them in the UK, or sold their house and chucked their stuff. Like life in any city can be rough at times and yeah you want to talk to your mum, don’t ever encourage your kids to put on a brave face when you’re already apart.
ALWAYS check in with your kids. Better they tease you for it than they sit there doubting you care. Make sure he knows it’s ok if he wants to leave London and it wasn’t the right move - now or years down the line. You have a massive role in his life, and honestly it seems a low bar but it’s so nice to read how much you care - keep it going!