Which area are you in?
His mock score is probably a good indication for bucks, but in Stroud and Gloucester area you are in super selective area.
My suggestion would be to go right back to basics with his reading, choose a shorter book for younger children with hard vocabulary, such as one of the pullman gothic series, brilliant for vocab: ( firework makers daughter, scarecrow and his servant etc) or some Anthony Horowitz horror, (gruesham grange 1 and 2) read it with him getting him to stick a post it note on a page when he does not fully comprehend a word, at the end of each chapter, go through the words and make sure he has picked up on any inference too. You can use newspaper articles at dinner time too.
He obviously can read all his normal books as well, but this is an exercise in comprehension and vocab.
My eldest often read books guessing his way through bits he didn't fully understand....My dh still does, I swear we cone to different conclusions about the same book sonetimes and I suspect it is because he read it half asleep.
Also I went through fptp cloze word bank on about four of them with ds to check he knew the words then let him fill it in afterwards with his newly learnt word definitiond written down on paper, I made a copy of the cloze just for him, so he can have another go timed over the Sumner. This was a very constructive exercise, rather than scoring low, then correcting mistakes.