I am sorry to hear about this, Mrs Skint. I know how much you love your pets.
I'm probably going to get really slated for this, (puts on flame proof suit) 90% of animal/human related problems come from not understanding the nuances of animal behaviour. That's not the owners fault, but people forget that the genetic basis of all dog breeds are specific to the job they were bred to do. In a BC's case, it was bred to herd.
It sounds to me that your BC is not so much being aggressive, but has a misplaced sense of where he stands in the pack (family) hierarchy. Most Border Collies do not make good family pets unless they have a job to do - my 2 are kept busy from dawn until dusk doing "stuff", and they are also very clear where they stand in the pack. (I'm the boss).
From what you say, It seems that your dog considers the kids as below him in the pecking order, and therefore he is disciplining them in the same way he would tell a puppy off. Your son clearly did something he disagreed with (got up), but you do was telling him to stay where he was.
PTS is not the answer and you are doing the best thing by trying to find a rescue space. In the meantime, give Wiccaweys Border Collie Rescue a ring (do a google search for their phone number, they're based up in Rolleston) and they can give you some free behaviour advice and suggest a few things to try whilst you are waiting for a rescue space.
Good luck, and I am so sorry you are in this situation, because I know how devestated you must feel.
RC