Charge him £200, put £50 a month in an account, when he moves out give him the savings to help set him up.
I've never lived with my parents since I left school, so have no benchmark to gauge against. But that said, the above is about where I would be. Except I'd be charging £250 and banking £100 for his first mortgage deposit.
He'll need a dig out for this anyway, so I would do that. I wouldn't want to make it too cheap for him (i.e. charging £150) because it wont be a fair representation on the costs of living in the real world (that is, afterall, council house rent without bill or anything), and if he knows that, then he is less likely to leg it (assuming you want him to go out and find his own way of course).
That's pretty much the route I'll take should this happen to me in the future. However, I really do hope that my children want to get out there asap and stand on their own, as I did, and their mother. Of course we will support them whatever, but you have to start showing them living costs money.
All that said, if he is a sensible chap, and is saving hard for his first place anyway, all you are doing is guaranteeing that first £100 per month saving. I also would not, under any circumstances, let slip that's what you're doing either. Make it a proper surprise. Even if he turns round to you and tells you he has the deposit together for his first place, there are always more costs in buying a house over the deposit, as well as furniture etc...
If you don't need any money at all, then bank the full £250, and give him half for his first deposit, and take him on his last family holiday with the rest.