Thank you all - many thanks for such rapid responses.
I know the solicitor thing is a daft idea but I've been so desperate for an intervention.
Yes, DS is repeating a year as he didn't do a stroke of work a couple of years ago. Things have improved - he's certainly very capable - but he's not really putting in the hours. He could so easily work for even a few hours a week and take some of the strain off us.
Yes, you are all 100% correct and I take entirely the point that by caving in each and every time (sometimes after a discussion, sometimes after a bit of an argument and sometimes just caving in because I'm too tired to do anything else) I am 'infantalising' DS.
I agree with a poster about the protein shakes - they aren't necessary and I think some of the ingredients are a bit dubious. He wants to put on weight - be a bit more muscular - but he's actually in pretty good shape and health and I have said that if he's really worried, then a word with his GP in the first instance would be the way to go.
Yes, there are cheaper gyms - this one is pricey and he has the kind of membership that lets him in anytime. He could down a level or, better still, go to the local college where the gym is much cheaper. He could pay for that himself or at least do jobs around the house and cook his own food (preferably ours, too, once in a while).
DH I mustn't be too hard on. He would like a more stable job and, in addition to his day job, he cleans at weekends. I'd just like him to be stronger. But actually, he's tired too ...
So, strategy - together we should be telling DS that the gym, as the membership stands, and the protein shakes and all that additional food are simply unreasonable and unnecessary and pointing out, as you've all suggested, that when he's at uni, has a job, a family etc, he'll have to budget, prioritise and sometimes say 'no' to himself/his kids. So why wouldn't it be the same for us?
And put in place a plan of action which involves him helping in the house and generally being a lot more resilient - again, a life skill, surely.
Yes, I have posted before. I'm ashamed that I'm posting again. I tried last time, and was encouraged by all the comments, but I failed. But now, right on the breadline, at least until payday and then the need, as ever, to constrain spending, and worn out, I'm going to give this another go. It motivates me hugely to know that I'm helping DS, whom I love very much, in the long run.
Thanks all very much - really.