Ok. Forgive my less than delicate language that is about to follow, but here goes.
In my opinion, merely finding his DNA (spunk, spooge, ejaculate, semen, whatever you want to call it) in his car would not be enough. I have more than one guy friend who likes to overshare and has told me that they have, at some point, engaged in a moment of self-indulgence (masturbation, wanking, hanging out with Rosie Palm and her 5 finds) in their automobile. If it's DNA evidence in the car that got the arrest warrant, it would have to be from Hannah. (In my opinion.)