Throw the text books away or use them to prop up a leg on a wobbly table.
Strengthening timestables is always good. Those can be done anywhere, no need for a book. That's what car journeys are for afterall.

In year 3, I'd be tempted to practice his number bonds and mental arithmetic (addition and subtraction) as well.
As for English, there realy is no substitute for reading. Pick a genre he likes and start from there. Make sure it isn't too difficult for him or it will become a chore. Don't get hung up by the "so called" classics. If you can instill a love for reading you have cracked it. If he is visiting the local library with spring in his step then you've cracked it. If you can't go past a WH Smith's without spending a small fortune on the latest kids blockbuster then you've cracked it.
Support his primary school and all the good things they are doing. Don't make the mistake of thinking (like so many parents do) that enough accademic work isn't happening there.
Come back at the beginning of year 5 and look at the formal text book approach. If you think he needs more than a year of prep then he probably isn't going to thrive at a grammar school anyway.