My parents gave me £50 a week when I was in uni, but I had some of my loan left over after accomodation and worked the odd day as an ambassador too which helped. I think if he has no loan to top him up, I'd be saying between £60 and £70 a week. £50 a week is doable (as I learnt when loan run out) but not very fun, so if you can aford it, I'd give more. Not for him to be frivilous and piss it up against the wall, but for him to be comfortable and able to buy the odd luxury now and then. I will agree with a pp in that I think dc are likely to face enough penny pinching in later life and that it's miserable to do it when you don't have to - I wouldn't want my adult children to go without heating etc in uni so they'll appreciate it when they can afford it. I also agree that those on the minimum loans get less financial help even though their parents can afford it. In my experience (and I accept it isnt the same for everyone), the students who recieved the less loan got very little help from their parents while the parents themselves had plenty of luxuries. Wheras students from poorer backgrounds, their parents tended to go without so many luxuries themselves to make sure their children lived comfortably and weren't scraping by. In the case of my own parents, they didn't have much when they were younger and wanted to give me a different experience. It didn't mean I worked less hard for my own money, because I had a few different jobs through uni - but it did mean that I never worried about feeding myself.
I think tbh what you give depends on whether bills are included or not - if in uni accomodation with bills included in rent then it's unlikely they need £100 a week. If they have to pay broadband, gas, electric etc themselves then I can understand the need for more. Really don't think they need £100+ disposable income though!
Basically, massive long rambly post but I'd say give what you can afford, within reason, you know your ds and what he'll spend it on, there's so many factors to consider. Perhaps sit down and ask him how much he thinks he needs - and ask him to explain his reasoning. Provided he's decent and not taking the mick (which he probably isn't), the figure he suggests will likely be close to what he needs. Give him a trial run of buying and cooking his food for a week off the amount he's set and that can help to decide what money is actually needed - could he compromise on some brands, is some food being left uneaten, could he have really done with a few extra quid for some more yogurt eg.