Is there any reason why you can't charge him ??25 a week, or ??50? That would be on a par with what other posters are saying they charge, around the ??200-240 a month mark.
You have been remarkably evasive as to what you are actually charging him and on what terms. From what some posters have projected, the rent goes down if he only works 10 hours a week, and so would only be ??95 a month, which is less than the "steaming shit" proposed by your ex. Your figures don't seem to correspond to your image of generous charity in providing your son with a room.
You also swing between not seeming to like your son very much (to be fair, if this was a man talking about his daughter in the same way, opinions would be very different on here) and saying he isn't ready to move out. But how is he going to save a deposit for a rented room in a short space of time?
You're also in cloud cukoo land if you think a room in a person's own rented family house is equivalent to one on the open market, where the landlord will have to pay tax and other requirements.
Its my experience that those who had it fairly easy themselves are the most keen to take money off people for living at home, and I think you don't want to lose the revenue stream from your now adult son. The way you talk about your ex is horrible (yes, things no doubt happened but (a) no child wants to hear that and (b) his email actually made some relevant points. You sound almost gleeful that this young man has a choice between staying in your house and being made homeless and that your manoeuvring has let you get one over on your ex.
I suspect most posters who think the same are now too scared of the traditional mumsnet vitriol to post, so I'll bow out now. I hope I'm wrong and the way you write gives entirely the wrong impression of you and your relationship with your son. But surely you have to be planning for him moving out in the near future, the way things are, and doing everything you can as a parent, to help him at this crucial stage in his life (and I don't mean giving him money).