The majority of Dothraki was indeed created for the show.
George has stated the he is not quite a linguist in that sense (comparing himself against Tolkien).
I don't have a whole imaginary language in my desk here, the way Tolkien did.
Tolkien was a philologist, and an Oxford don, and could spend decades laboriously inventing Elvish in all its detail. I, alas, am only a hardworking SF and fantasy novel, and I don't have his gift for languages. That is to say, I have not actually created a Valyrian language. The best I could do was try to sketch in each of the chief tongues of my imaginary world in broad strokes, and give them each their characteristic sounds and spelling.
So Spake Martin, YET MORE QUESTIONS, July 22, 2001
He mostly just makes words as needed.
"DO YOU HAVE THE LANGUAGES ALL PLANNED OUT?"
No he'll make up a word here and there but he does NOT have the whole language written.
So Spake Martin, BOSKONE (BOSTON, MA), February 14,2003
Even going as far to say that now he will have refer to the creator of the language for the show in his future writing.
Of course, he then added that with HBO having created Dothraki through the work of David J. Peterson, he feels like now if he wants to have Dothraki (and Valyrian as well) he'll have to refer to Peterson's work to get it "right", or ask Peterson himself how to say something in Dothraki.
So Spake Martin, STOCKHOLM AND ARCHIPELACON REPORT, June 28, 2015
The show selected David J. Peterson, an expert language creator from the Language Creation Society to create Dothraki for Game of Thrones
Peterson drew inspiration from George R.R. Martin’s description of the language, as well as from such languages as Russian, Turkish, Estonian, Inuktitut and Swahili. However, the Dothraki language is no mere hodgepodge, babble or pidgin. It has its own unique sound, an extensive vocabulary of more than 1,800 words and a complex grammatical structure.
Which as of September 21, 2011 has over 3,000 words
Also, for the curious, Dothraki is now up to 3,163 words: More words than Mr. Padre Tony Gwynn has hits, but still a ways to go to catch Pete Rose (and my guess is Dothraki will have double his number before he gets even a sniff of the Hall of Fame).