How do you expect him to save enough for a deposit and first month's rent so he can move out if he is paying you 1/3 of his salary each week, from the very start?
I mean, by all means, make him pay his way, make him independent, but don't trap him. Shoplifting, erratic attendance at college, his father doesn't want him, a poor relationship with his stepmother - he sounds very troubled. But he has found a job quickly, so why not let him see the benefits instead of totting up how much money to compensate your loss of child benefit and maintenance you are getting?
You can also get a room in a shared house for £250 a month round here, including bills, and would have quite a choice for £350 - £400. But you need a deposit. If you want him to be independent, why not do something for him to help him? You could work out how long it would take for him to save a deposit for a room, plus his first month's rent, say £1000 for both, then maybe another £500 for removal expenses plus all those things you need to buy when you move. So £1500. Let him save or take that from him as a fund and don't keep it yourself and then if he is still there, charge him 1/3 of his salary in rent from you.
I wouldn't worry about him spending his first two week's salary on himself. Seriously, who doesn't? Didn't you? I was delighted when I got my first paycheck and probably spent it on something stupid that I can't even remember now.
I mean, yes, he has to be independent, but parents give their children so much help now. I have a flatmate whose mother has basically stayed with her for the first week, seemingly so she can complain to the landlord about every tiny little thing. At the moment, it probably seems to him that neither of his parents want him very much and he is thinking his dad might help him out at last (probably a forlorn hope). My parents weren't very great either so maybe I'm more sympathetic towards him. I certainly wasn't rushing back to visit them after I felt I had no choice but to move out at 18, and I still feel envious and a bit sad of people like my flatmate, whose parents not only give them money to support them but time and effort.