If he's decided not to continue in further education, but to go out to work, he is now part of the adult world, full time.
I would tell him that it's not unreasonable now to expect him to move out into his own flat, and start looking in your local paper/estate agents' window for flats to rent, and working out with him just how much it would cost him.
My nephew has his own flat, with his girlfriend, in Leeds, and they pay £500 a month just for rent. That's before any bills. (Oh, and we're not talking about a swish, City-type, loft-style apartment, just an average/small 1 bedroom flat).
You don't, of course, have to insist that he leaves home, but it will illustrate to him just how expensive it is to be an independent adult.
This will do two things. First, it will make sure he is well aware of the cost of living and how much he needs to do to make sure he can afford it. Second, it will make him appraise whether or not he has made the right decision, and whether or not he might actually prefer to go back into full-time education.
Those of you who refuse to take any rent from your children are ensuring that they become those who have, to use Mumsnet Bingo, a massive sense of entitlement and who will not take any responsibility for themselves, just assuming that other people will pay their way.