This has been a really interesting thread. I'm sorry if I didn't make the situation completely clear to begin with: I see how the confusion arose. But it's quite good it did, too, because it's thrown up lots of issues and I think what we're really talking, from the mix of reactions, is setting clearer boundaries.
I disagree that he's 'working' in an employment sense and 'getting less than the minimum wage', as really we made a deal we were both comfortable with - however weird it sounds, I give him a place to live and study while at the same time for us he's a sort of temporary member of the family - a feelgood factor to help take the pressure off me and make the kids feel less like they have this strung-out, wired mad single mum trying to juggle everything. A kind of rent-a-bro who can tell them where Timbuktu is or fight over the rest of dessert with.
Also their Dad is causing no end of stress and misery, and this guy helps by being simply a positive male presence. I have three girls and don't want them growing up thinking all guys are aggressive and unreasonable.
His rent-a-bro 'obligations' are only on four nights a week (btw often the kids aren't even home for the first couple of hours so the 16 hours is a very generous description of how much time he really does have to spend), and if he had any reason to swap around etc I'm flexible.
Also,I think if I did have a son his age (25), by now I'd be expecting him to make some kind of contribution to the household somewhere down the line. The problem is, a student is a dependent so that makes it trickier. I certainly don't want to fleece him: no way.
BUT on the other hand, maybe this is really a bizarre arrangement! Rent-a-young-man...I'm 47. I admit, not everyone would get it. I personally have no problems with him being with the kids: he's great with them and they make each other laugh.
For what it's worth, everyone seems happy. I'm nervous sometimes because I've let people take advantage of me in the past. I think I have to be clear about which food I need to save for certain meals and that's the bottom line.
But thanks, all of you, for your views on this. 