@Rickrollme
I would encourage him to try to make London work if at all possible. At such a young age he can share a grotty room and eat ramen noodles but there is no substitute for being in the heart of it all, among other like-minded young people. The people he meets and experiences he has could open up future opportunities that he will won’t have anywhere else. If it’s miserable he doesn’t have to stay but at least he tried. As he gets older his responsibilities will increase and he may never have this kind of chance again. I think 25 is awfully young to compromise your dreams if there is any alternative.
Absolutely this!
If you can't live in a shitty house share and survive off ramen when you're in your 20s, when can you?
Oh, but of course, this is Mumsnet, where everybody's precious children must live at home until they're 30, get a good job in the civil service and save a good deposit towards a new build in a commuter town...
OP, assuming your son is relatively physically and mentally robust and isn't expecting kittens and rainbows, he will be fine. Maybe send him a care package every now and again?
PS - I worked in a bookshop in my early 20s when I was doing my MSc and it was, hands down, the best job of my life (I say that as a senior professional in my early 40s). Paid money to read books, talk about books, arrange books and occasionally chuck a duster about.
Wish him good luck and many adventures from me. 