I think he his too. He's done a few other projects in the world, but not the 6th book. I think he was waiting to see how things end to keep his timeline intact. Since his chapter structure is completely different than most books.
Don't get me wrong, I love the series. The books equally. They are each their own. I think anyone who hasn't read the books needs to. The Red Wedding is one of the best points as to why. In keeping with the book community there, no spoilers from, except there are some major differences. Both still succeed in shocking the audience.
Yea, hopefully he will manage to complete the book series !