From what I've read, the series was done when the first book was published. He and his editor then went to work on editing and revising the second book and some of the changes ended up being pretty substantial, hence the delay in publication of book two.
With all of the changes to book 2 he now has to deal with the huge butterfly effect on book 3 and it seems like he can't quite pull the trigger on scrapping it and starting from scratch. Instead, I've gotten the vibe that he's trying tweak everything in the remaining framework that is book 3 and is stuck adding, subtracting, and rearranging everything so it aligns with the current books and holding to the promise he made to the readers on the original back cover. He's fighting the never ending battle of fixing plot point H, adding I, adjusting R and realizing that A and B no longer make C relevant.
He needs to scrap everything, start completely over, and tell the fans to check back in around 2020.