Thing is, he can get a loan for his tuition fees (of up to £ 3,225, which will be the normal level for most subjects next year)which is not dependent on family income. It's paid back on the same terms as the maintenance loan and, like that loan itincurs interest at a level based on the inflation level - it's not exactly inflation as they only set it once a year, so sometimes it's higher, sometimes lower.
So it might be easier to let him take out the loan for that & use your contribution to help with his accommodation, because his maintenance loan, which will be around £3,500, will not cover a lot - DD is currently in a catered hall (only because if she wasn't, she'd never eat!) and her fees this year are £3600.
Living in a self-catered place or in a house is cheaper, but you have to buy food & services - DS pays £265 a month for 11 months and has to buy food, utilities & anything else on top. One of his housemates only gets the non-assessed part of the maintenance loan, but her parents say they can't afford to top it up, so she has to work probably more hours than is good for her coursework, as she's doing science & has to put in a lot of lab hours.
We pay DS & DD's accommodation, and they use the maintenance loan (like your DS, they only get the 75% non-assessed part) for books, travel etc plus of course food in DS's case. Because DD is getting her food paid for in Hall, she is supposed to be saving the extra, and so far seems to be budgeting pretty well.
We are comfortable with this level of support, though this year is a bit tight,but hopefully this will be the only year when they are both at university, and they are well aware that anything we pay now is coming out of their inheritance!