Hi
I tutor in English and cover comprehension at this level. IMHO the Bond books are not strong in their comprehension training, in comparison with their other resources. Their questions are often ambiguously worded, the answers restricted and at times confusing, and the passages are dull!
Are you good at comprehension? If so, read with your son, and stop every couple of pages to ask him questions, checking if he can work out things that are implied or foreshadowed in the text; if he can work out what a new word means from its context etc. I read to DC every single night in yr 5, from a wide range (usually fiction). We kept a notebook and wrote down all vocab they didn't know then used it often in conversation, sometimes jokily. They both did very well in 11+ at schools of similar academic strength to Colet Court.
Read aloud from authors who challenge young readers: Eoin Colfer, Cornelia Funke, Philip Pullman, Morpurgo, Joan Aitken are all good at developing vocab. Louis Sachar is easier to read but great on story structure and moral concepts for discussion. Hampton school used ot have a great fiction reading list to prep for 11+ on its admissions pages. Not sure if it still does. To familiarise him with vocab of old-fashioned extracts, we used poetry more often as it's shorter and easier to access. Old Possum's Cats by T S Eliot and the dramatic monologues of Robert Browning, short bits of Shakespeare (the rude mechanicals or the Macbeth witches) as these ease them into confidence with archaic language which is sometimes sprung on them. Also make sure he reads a newspaper - even Metro or ES are OK but the I, compact version of The Independent is good for factual reading and structuring.
To increase vocab, play synonym games with him:
how many words can he think of that mean shining (glittering, sparkling, dazzling, glinting, gleaming, polished, burnished, glowing etc) or smell (scent, perfume, stench, reek, pong, aroma etc) Encourage him to reach for a dictionary when he doesn't know a word. Use long words in conversation with him and check if he got them.
We also used to play alphabet games. Pick a subject (eg animals, fruit) and then go right through the alphabet finding an adjective and noun for each letter. It can be silly (ambitious apple; boisterous banana) as this encourages confidence with language. Play scrabble and banagrams and make words he doesn't know so he can learn them when he challenges you.