I am tutor, and I sort of agree with what she is saying, but recommend doing it like this.
have a notebook and pen, when you find a word you don't know, jot it down, quickly and quickly explain it enough to go on with the story. At this point the flow of the book is most important. If it is your book you could underline it in pencil in the book.
Then at another time look at those words, look up meaning, I recommend googling it. Scroll down the meanings until you find one he understands. Write down the meaning, but write down synonyms and antonyms as well, as they ar eusually easier to grasp the meaning of the word, and a couple of example sentences.
Deal with no more than 10 words at a time, and review them over the course of a week. You could have a prize jar/chart, eg anyone who manages to use one of this week's words in normal conversation gets a sweet out of the jar.
As the words get learnt, add in new ones, so you should be able to learn 10 words a week.
I disagree about books. Joan Aitken is very dated, and hard to read for most kids. A good ruel of thumb is the one hand rule. Read one page at random, and start with a flat hand. For every word that you don't know/can't easily guess on context fold down one finger. If you fold down all the fingers of one hand on one page it is porbably too hard to read ot yourself (but may be a good one to read aloud)
The biggest aid to learning vocab is making sure he really does read and read and read, every day.
BUT he is bilungual. That means that if a typical 8 year old has a vocab of sya 3,000 words (I have no idea what the actual figure is) and his English vocab is a bit low, eg 2,500, you must remember that his actual vocab is double that, as he knows those words in English and his other language, so he has much bigger language than his contemporaries, around 5,000 for example. The challenge is to make sure he has a broad vocab in both, and one thign that can happen is that he might develop 'school words' vocab in English and 'home words' vocab in his home language, so it requires a bit of effort to keep it borad in both