This is such a tough one! It says an awful lot that he's prepared to wait a year to get what he really wants - but there's no guarantees next year...
I would have a good talk about what he would do in that year, to make sure he's still learning and has the same focus - so that when he auditions again, he has something new to offer (and has, of course, enjoyed that year too!).
We know a super talented person who went off to ArtsEd, having been offered a foundation year there, it being her dream school. They were offered the full degree course elsewhere but it was their dream to go there so they went that way. A few months later they were home, it just didn't work out. A year or two later, having regrouped and plucked up the confidence again, they are now sitting on at least two offers to start again (full course) in September - I am sure ArtsEd is one of them. Sometimes time, circumstance and where your head is at all play a part.
I can only say if my 17yo stubborn DS were telling me he would rather wait a year than go to a certain uni (non performer), I doubt I'd persuade him otherwise - and if I did, he'd have a predisposed filter to the experience... So as always, you can only let him make his own decisions I guess! And not everyone is as downright stubborn and cocksure as my eldest!
Look into what happens if you take the other offer but then the reserve place is offered, and use that to decide - you can wait till whatever the deadline may be I assume...
Good luck! Well done to him, he clearly has the talent!