My children enjoy maths, but have often been bored with maths at KS2 - their school sticks fairly rigidly to the curriculum.
I have been reluctant to push them towards KS3, preferring to challenge them with the maths they already know. I am always on the look out for mathematical style puzzles that I think they can do; this does require quite a lot of effort.
For maths word problems, the MA publish a book called "Challenge Your Pupils" which contains lots of examples of questions from their "Primary Mathematics Challenge". To get an idea of what is in it, I think they have some 'past papers' on their website. I liked the questions, but would prefer not to have multiple choice answers.
I prefer this book "The Complete Book of Fun Maths". It is aimed at adults so you will need to try them first and pick out the ones you think he will be able to do. Amazon's 'look inside' feature shows a good chunk of the first chapter so you can get an idea.
"The eleven-plus book, genuine exam questions from yesteryear" contains lots of wordy maths questions which I found good - and more challenging than the stuff Y6 kids seem to be given these days.
There are lots of classic maths puzzles that he may like. Things like the 'fox, goose, corn river crossing puzzle', 'fixing a broken chain at minimum cost', 'switching positions of counters'. The murderous maths site has some great ones eg 12 coins problem If he really likes these puzzles, you can find whole collections of them in books such as "Amusements in mathematics", "The Moscow Puzzles", some of which are old and available freely on google books - will require lots of effort to find ones at the right level though.
Logic puzzles of the type you get in newsagents (the ones with grids) might be good for him. Mindware publish graded books of these and call them 'grid perplexors' - expensive for what you get though. "The Works" often has cheap books of this type along with IQ / brainteasing books.
No suggestions has to how you can help get the most out of his school though I am afraid. Unfortunately many primary schools fail to recognise that some children need more of a challenge than KS2 maths offers - imho.