Sympathy vibes flying your way OP. Is supernanny doing a teen series yet? Please hurry up if not Jo.
If you are screechy and/or authoritarian, they will defy you as a matter of principle. Don't ever get upset or you've lost from the start. I was an horrendous teen (apparently) but there was some stuff going on at school that nobody knew about and I didn't know how to tell anyone. Agree that the move to secondary could well have a bearing on how he is behaving - you could try to find out if all is well on the school front? (don't do it obviously though). ime the teen years require totally different parenting skills and the shift can be very challenging for the parents, threatening our sense of being in control/in charge. There is a decided insurgency on the part of the 'teen' too - peers can also have a huge bearing on their behaviour. They can hugely undermine family peace, causing tremendous stress in all family members - though they can't see it and, anyway, aren't interested. Teens are almost autistic sometimes, totally one track (or maybe two if you're lucky) - this is probably neural in origin iyswim, the body going through vast changes, the brain lagging behind.
ime boys generally respond well if you're doing something with them, and don't respond well to talking. Find some things you can do together - preferably fun initially - and take time out for one-on-one. You don't have to be aiming for huge breakthroughs or addressing The Problem (him, his behaviour) but literally one-on-one time together, even if the convo is sparse (which it probably will be), even if it's for half an hour bursts, though hopefully aim for longer. I agree that packing a suitcase is a task too far for a teen boy (or pre-teen boy in your case) - he probably has no idea where to start, no idea of planning - and, anyway, isn't interested. Don't make him feel stupid for this - he probably feels stupid enough already. I think that cancelling the trip may be too harsh but impossible to say in the circumstances. The one thing you do need to do though is stop shrieking (even internally) and getting upset - this is petrol on flames. You have to be a rock, immovable, unflappable, a plank of wood, calm, pleasant.
Impossible to cram insights I may have gained in a few paras though - we're all different, all our families different etc anyway. ime the teen years are like a hurricane at sea: you put in place the boundaries, enforce them (even though your voice seems to be lost in the howling winds) and generally batten down the hatches until it's over. You'd be surprised how much they hear, how much goes in. If all they're hearing is negative they try to tune out but, actually, get quite despondent at the repeated message that they're totally crap in every possible way. They respect us more than we think or is obvious. They also need us much more than they or we realise. They need boundaries like a dying man and it is essential they get them and that they are enforced.. with kid gloves. Good luck OP.