I got this book a couple of weeks ago based on a recommendation here, and it has become something of an obsession with DS4: You Can Count on Monsters. It won't teach him math facts, but it is definitely a book about multiplication (and prime numbers). DD8 likes looking at it, but DS4 loves it. We read it over and over and over, and talk about the monsters, and recognize the different ones, and count the dots to do the multiplication. I expect that he will be totally solid on multiplication facts just by osmosis after he's been into this for a while.