DH has said he will not be allowed to sit round the house all day if he doesn't get to uni; he will be expected to be out looking for work.
And what will DH do if DS does just sit around the house all day Horsemad? Put him out on the streets? Find a job for him and drag him there by his ear? I find it's best not to make sweeping threats that DCs know won't be followed through. If you make threats like that and go on about things too much it just becomes meaningless background noise - always there and easily blanked out.
X-box was, I am quite sure my DS1's downfall. DS2 has never really been into gaming - much prefers to be out on the rugby pitch. But you can't turn them into something they are not sadly.
I know backing off is easier said than done, and, I was, at the stage your DS is at, incapable of following the advice I am giving now. But I would guarantee that, if he does turn things around over the next few months (and that's a big ask at this stage) it will be because he has decided to - not because of anything you've done.
There is always the concern as well that, whatever the outward appearance of bravado/nonchalance, they are eaten up with fear and just don't know how to cope. I will never forget shouting at my very placid DS1 who never seemed to respond to any of my 'advice' and him turning away from me with tears in his eyes. He clearly felt deeply unhappy about his predicament, just couldn't fit into the mould I was trying to force him into. He's a much happier person since I backed off and let him find his own way.