Arrrrghh difficult. I thought you were going to say that finances were really difficult at home or something.
I do understand why you might resent supporting somebody to have 2 1/2 years of laziness and worse when working extremely hard, dealing with mortgage and fuel bills escalating, and living off beans on toast yourself, so that they can have a party lifestyle.
Maybe have an open, kind talk to find out what he thinks life might be like at 21, 25 and 30. Also remember they'll grow up and change a lot. And a lot of university is about the networks they make and learning to be independent.
If he does go - nudge him towards worthwhile programs with pathways to good graduate careers (e.g. go to open days at places that look promising, to plant the seed).
And think about what your lines are. If they aren't prepared to look into what they would be a good fit for, and identify reasonable places to study - well-located with good housing availability - then they aren't signalling that they are being serious about it. Then, you're not being as unfair as you would appear to be if you said 'you're lazy so I'm not paying' without saying explicitly what it is you need to see as commitment.
If going to university is going to be a non-starter, there are other ways to have a student-type lifestyle - e.g. gap year and travel before a degree apprenticeship (though those are often really competitive). Life doesn't suddenly become party-less and friendless at graduation or on taking up a proper job.
But - even if not going to university, it would need to be a developable job unless he wants to end up being paid the same as a 21-year-old at 30, and resenting being paid half as much as a 25-year-old graduate at 40.
Really tough for young men and their parents at the moment - many are just not ready at 16-17 to think seriously about their futures - I feel for you.