On the Verbal Reasoning paper, the paper for which he missed out by a few marks, he had 9 sequential questions incorrect, apparently within the same section. I'm thinking he read the question wrong, as although they don't say what type, they do say that it largely related to two responses being required for each answer - so possibly opposites or similars.
Although I am not familiar with the Slough papers, I fee fairly sure you are right - in Bucks these two question types (opposite and closest meanings) are the downfall of a lot of children, simply because they fall for traps. There will be closest meaning options among the opposites and vice-versa.
My question is how do I include this in my evidence, and how do I say this in the best light that would help our case? Or should I not mention? Had he got this section right he would have passed.
I would mention it briefly if it appears that he was indeed very
sound on all the other questions, but it won't hold water if there were other slip ups.
You can simply say that "it appears to be a single question type that let him down in a moment of confusion". It might elicit some sympathy from the panel, but you do need to concentrate very firmly on demonstrating high academic ability otherwise.