If he isnt moving from the brambles is he caught up in them?
How long have your sheep been missing? The one in the brambles is almost certainly
stuck - as I have said before, brambles are carnivorous and it sounds as if your wether is the next victim. Sheep are unable to disentangle themselves and will stand until they die, so there's nothing for it but to shoot him or go in and get him out - you can't just leave him there to starve to death. You will need help to fight your way in and someone to hold him while you disentangle him or he will be off the second he feels he is free. Once you have him, keep him in as he will need rehydrating and feeding up. He may need to see a vet to check his lungs for pneumonia.
For the other one, have a look at other sheep flocks - he may have joined one just to be near other sheep, even if they're not quite like him. He also might have been hit by a road vehicle.
Finally, find out where they escaped and make your fences Soay proof