Is he home educated?
If not, why not start by encouraging him to pay close attention to everything that he is being taught in school, before worrying about getting a few things wrong in papers designed for testing above his current year group. Particularly important if it is the basics that he is falling down on imho.
If your DS genuinely enjoys maths puzzles as puzzles, so to speak, then the nrich website (which you have already found?) has lots of them, also the Primary Maths Challenge site
http://www.primarymathschallenge.org.uk/