If you say "He QUOTED you the price to lay STONE flooring" then A. Travertine is stone so he can't get out of that one and B. Quoted you the price,not an estimate then he has to honour the price by law(I think).
I personally would never quote until I knew the tile,as said before though travertine is one of the easier stones in my opinion.Its not like you've turned up with uncalibrated slate.
Most builders aren't tilers,as said before do you want someone whos not confident laying your expensive tiles,its too late once they're down.Make him pay for a tiler.
Good luck.