It's a sticky subject to understand, but I'll try and help out.
Even though he doesn't have each and every tailed beast as a whole sealed inside of him, he still has their chakra (or in more theoretical terms, their essence), which... is pretty much the same thing. He can't access their FULL power - but they can still lend him power here and there to aid him in his battle(s). If you read Naruto 675 when he forms a Sage Art: Magnet Release Rasengan, you can see one of the chakra balls behind him distort. My guess is that each ball represents a tailed beast and their power; which means he should have nine total, but from what I've been reading on different forum websites, he has 10. As for the 10th, I have no clue.
Does that help clear things up a bit?