I think you are over reacting in the sense there is no need to make a drama out of the whole thing.
If your son definitely does not want to go, is getting anxious and crying over it and you do not want him to go then it is really very simple, isn't it? You simply meet with the head as requested and explain that you do not want your son to go, in part because your son has decided he does not want to go. You could ask if the school could take a deposit, in case your DS changes his mind, and if the head teacher agrees then take that path. If not, then simply decline to send your son. No need for a fuss, the school can't force you to send him.
I agree it is important to listen to your child when they express an opinion on these sorts of things. I wouldn't force a child of that age to go on a residential trip if they were dead against a trip after suitable consideration. However, it is also right to encourage them to think about these sorts of things and not simply go with a gut reaction, to talk things through and explore their worries/fears to see if these are well founded or not, and to see if there is a reasonable way to overcome/tackle any problem,s/worries (where that is sensible). I also think it is important to encourage children challenge themselves, within reason.
I think it is also sensible for parents to acknowledge where our own anxieties and worries can rub off/be communicated to our children. It sounds like OP and her husband have quite a lot of worries/anxieties of their own regarding this trip and wonder if this has been communicated to their DS.
I find it very odd that children would leave the school sobbing about the trip. Mine was ridiculously excited about being given the option of going on a foreign school trip (same age) and even her friend who was far from keen to go was not so upset at the prospect of an optional trip.