He's 72 years old, hasn't written a new word in almost ten years, and spends all his time doing fan conventions or developing new shows for HBO. Where would he even find the time to write? I think he's just lost interest.
Certainly more likely than him actually finishing the books himself, at any rate.
Not to my knowledge, but on the other hand-- broad strokes of the plot aside, he doesn't seem to know how it's supposed to continue from here.