Judging by the trailer, Trevor probably gets trapped/converted/whatever once he meets dear old dad. When Simon decides to trek on in himself, he encounters/awakens/maybe even fights a possibly brainwashed Rovertucard and they enter some sort of partnership.
Smells like CVIII, only with Trevor going all Alucard and Simon taking the role of... well, Trevor. I wonder who the other two playable characters will be and how they will compare, assuming that Alucard-Trevor isn't one of the 4.