There are going to be seven books total, and he's only finished five so far.
He did sit down a few years ago with the writers and tell them the "big picture" of how it was supposed to end. But he's also gone on record saying that he unhappy at a lot of their storyline decisions.
https://www.express.co.uk/entertainment/books/1123230/Game-of-Thrones-George-RR-Martin-HBO-ending-books-change-differences-Iron-Throne-die
he should have got busy faster if he wanted to control the narrative or put it in the contract. Once the series started they were headed down the track and on a schedule.