Oh your poor lad and poor you too, that's absolutely outrageous.
Swimming is part of the curriculum, but we were told we didn't have to give permission if we didn't want to - not sure how legally enforceable it is, but the DfE website states this.
My ds has a phew minor physical problems as well as sensory issues around water on his face and most of his course was spent trying to get him to cope with being in the water and getting his face wet. Like your ds, he has always been terrified of swimming, but wanted to give it a try with his classmates and it was a very bumpy ride to begin with. He did do the 25 metre assessment, but only with armbands and floats and an adult in the water alongside him - so essentially he did not meet the requirements of the curriculum.
As saggarmaker said, swimming has to be differentiated for, just like any other aspect of the curriculum and is therefore covered by the Statutory Inclusion Statement.
I would also be inclined to put my complaint in writing to them. Particularly how serious it was for them not to have his emergency pack and pointing out that you had specifically highlighted this as a requirement of him being allowed to take part. Blaming the outside organisation is not good enough - it is the school that has a duty of care towards your ds and they were responsible for making sure his needs were met. I would also inform them that you are If I were you withdrawing permission until a full risk assesment has been carried out. If, after the risk assessment has been carried out, they are unable to come up with a satisfactory plan of care for his participation, you would be more than justified in refusing permission.
If you feel you are not going be able to trust them again though and assuming your ds still wants to try again (and nobody would blame him if he never wants to go back) is there any chance you could go along yourself as his 1-1? I know our school is desperate to get as many parents to go along as possible, as it makes life much easier for them.
One last thought is, if your ds is now truly too frightened to get back in the water, could he possibly go along with the other pupils, to gain an awareness of water safety, without actually having to get in the water? You could possibly argue that that is appropriate differentiation, in order to meet his physical and mental/emotional needs. I know we had one child with behavioural problems, who wasn't allowed in the water for safety reasons, but was given the job of being the teacher's 'helper', so that he could still access some of the learning.
I really hope your ds feels better soon and doesn't end up needing the drip.