The question will always be there, because it's impossible to prove he's not. Testing has been exposed to be imperfect.
As has been stated, his rivals have either crashed out, made early sacrifices, or made glaring mistakes. He has has been both lucky and smart.
His tactical decisions on the bike are beyond what you'd expect from a rider of his age, and his team has done a fabulous job of keeping him fueled and hydrated.
I'm going to assume he's clean until proven otherwise.