If he is going to be able to cope by himself in a grammar school, he should be able to cope with reading a book without you being there to help... Give him a dictionary and encourage him to ask you questions if he'd rather, but otherwise save the comprehension questions for a more relaxed chat over dinner/on the way somewhere - are you enjoying the book? why/why not? what are the best bits? do you think your brother/friend etc should read it? why/why not? Easier if you pick books that were your favourites as a kid, so you can ask more specific questions and also show your affection/passion for the book, which is catching! I was so pleased when my dd loved Ballet Shoes as I had done, and enjoyed Little Women despite the huge gulf in cultural understanding (both geographical and historical). Don't underestimate your ds - he can read great works of children's literature (and so many of the best books written have been for kids) and enjoy them, as generations of others have done. Yes, he will be expanding his vocab/comprehension etc while he does that - but ideally that should be seamless. Literature isn't and shouldn't be viewed as a chore - either for you or him. It's a joy and a window into other mysterious, magical and wonderful worlds.
Sorry - got a tad evangelical again.