Thank you, all.
I've just read all the posts, twice - lingering over quite a number of them because I can truly see myself and the family dynamics in them. I do think that I've made a rod for my own back. And that rod is now heavy and wearing me down. I haven't been a push-over and I haven't been soft throughout DS' life but I do see that now, when it's fair to say that he's a young adult as well as still (obviously) my child, things must change pronto.
Other DCs left home - sadly, both were hopeless about taking responsibility and I suppose a culture of forking out developed. One of the other DCs is in his postgrad year and has only recently started to work part time - apart from the occasional holiday job. Not good but better late than never. As he has now has no loan, we have to chip in to help him but I'm actually less upset about that than I am about the DS who is still at home because he's living here, free, comfortable, well fed. His almost constant gripes about food (a 'fussy palette' as one of you so aptly put it) and his weight (Greenfizzywater - I suggested he saw the GP about his anxieties about his weight - I think he's over anxious about that) are very tiring.
This afternoon, he asked me about the protein powder again - in a moment of weakness and stupidity a few days ago, I told him I'd order it for him online. Well today I had to say that this hadn't happened because we're limping towards the end of the month - and come the beginning of the next month, the budgeting will start again. I thought of what's been posted here and told him that these things were extras - nice for him - but to be paid for and that even a few hours of work a week would go a long way. He became angry - lots of F words - and said that now wasn't the time to get a job (though he acknowledges that he had promised to do so when the whole issue of gym and powder and lots of extra came up). Alright I said, but he could find chunks of work in the holiday - he grudgingly agreed that he could but I got the impression that he wasn't going to do anything about it.
He said that the gym was essential - how else was he going to tone after consuming the number of calories that he needs - and that the powder wasn't but it was equivalent to a 600 calorie meal which, if he didn't have the powder, he'd have to make. (BTW, I do worry about the number of calories that he thinks he needs, and the amount of protein, too. Again, I wish he'd take professional guidance.) I stuck to my guns and said that as he wasn't able to contribute, the powder was out of the question and the gym was something that he should be paying for via work - even if was the chunks in the holidays (there's somewhere near here where he could work for the odd long weekend, for instance) or, at the very, very least, finding a cheaper alternative. The local college gym, he says, isn't suitable for him - but he's never visited it.
Things calmed eventually - I really tried to remain calm.
So, I've made a start. I really agree that no must mean no. And I like this from carabos (thanks, carabos):
"IME there comes a moment with teens when you just know they're trying it on. When you open your eyes to that and take back control, they tend to go "ok, fair cop, you got me" and either chip in or move out. Just in case you were wondering, that moment is now."
DS is, I think, going to be OK at uni. He had an awful first A level year and has picked up quite a bit. He has high predicted grades and just needs to get down to the work. The tutors are for the subjects he had difficulties with - and one he still does - it seems to be making a difference, though. I can just about cope with that - I've put their cost into my 6 month budget - but not the other stuff. He doesn't really go out (except to the gym) and I wish he did. But sadly he doesn't - he's found it hard to make friends in his new sixth form where everyone pretty much knew everyone else from secondary. He spends a lot of time on the computer (and, yes, like idiots, we fund some gaming sub - but that's not nearly as much as the shakes and extra food).
Strategy is going to include: computer curfews; reasonable household chores; no shakes (surely unnecessary anyway) and a cheaper gym sub if he pays for it/contributes through work that he could quite easily pick up for the odd weekend in the holidays. He has the time - witness how much time he spends recreationally on the computer.
Sorry, this is very long - but there were a no of questions and I wanted to answer them all! THANKS all - you have given me strength and hope.