OK I am one of the first to jump on and tell you Stifle is hereditary BUT just sometimes, it is a one off caused by injury.
You need to look very carefully round wherever he was immediately before he locked up and see if you can find any evidence of casting or being trapped... a struggle of any sort.
Sudden onset of this severity is not normal.
If it is an injury, the ACP and Bute (or whatever...Banamine is not so good for inflammation BTW) will help a lot, and trailering will not!!
It is a shame the Vet cannot come immediately, those of us in a position to just lift the phone cannot even begin to understand thsi situation, it must be horrendously frustrating, but you have done everything right.
I am sorry, I missed the actual details of the horse- if this is a foal at foot I would leave him loose in a paddock on his own with Mama.
If this is a weaned or grown animal I would stall him with his best friend next door, give him plenty of hay and cut down on the grain, and just keep him quiet.
I nursed a filly through the worst case of patella luxation I have ever seen and though I did have her euthanized at four, this was purely as I could find no-one I trusted not to breed her and to keep her correctly (she was a CrCr and a Big Horse), so I do know how scary this is, BUT my filly came to this slowly, she did not just lock up one day.
Now, many many years ago I had a Cob gelding who, somehow, got his leg stuck through a wooden gate, took it off it's hinges and sat on it!!! (only a Cob!)
After I had got him up one leg stuck out at right angles to his body.
I called the Vet.
The Vet stood and looked at him (before the days of ACP and Bute, back when Neanderthals still wandered past...no wait, that still happens...
)
Vet went away saying if it did not go back we would operate!!!!!
My little riding mare got "wiggy" with him over a pile of hay, bit him hard and he turned and put his foot down...must have hurt, and I nursed him for over a week, just hand walking and leaving him out in the field without the cross mare, but he never, ever, had a moments problem with that leg...went over to Ireland, grew a full hand on the grass there, and then went on to be a Hunter...which, believe me, in Ireland you have to be sound to do tha!!
So hang on, keep nursing, it is the best medicine in the world!!