"They have said that they will revisit his predictions before half term but I'm not sure how that works if his UCAS form must be in before then."
His UCAS form doesn't have to be in until January unless he's applying to Oxbridge or for a very few highly selective subjects (and I'm sorry to be blunt but he's almost certainly burnt his boats there with those AS grades).
He needs to talk to the UCAS person at his school if they have a school rule on very early applications for everyone, so that his new predictions can be taken into account. I'm sure they can cope with ONE kid putting their form in mid November for a valid reason.
He also needs to consider that if his predictions (and, far more important, his real grades) don't go up, he can either lower his sights, broaden the subjects he's prepared to look at, or not go to uni. He almost certainly won't get in to an RG uni with Cs and Ds, that's rather beyond "slipped under".
DD got one grade lower at A2 (ABB rather than AAB) despite working far harder.
Sorry if that's not what you wanted to hear :( But I do think you need to consider it rather than assuming those grades are the abberation. What made DD's A2 year bearable, especially the waiting for results part of it, was the DDD offer from a "second rate" uni in a lovely place which she knew was sitting there if she needed it.