What was there to say? She stuck her head out. He was speeding - for a good reason - and had to swerve to the side. It was a horrific accident.
And he doesn't "casually" go home, the whole emotional point of the scene is that he's too devastated and terrified to deal with the situation like an adult - to even look at the body, or call the police to the site or whatever - and he shifts the burden of actually discovering the body onto his mother. He does not have it in him to own up to what happened, which is what she calls him out on ("Nobody admits anything they've done!"), while she resents both him and herself for making Charlie go to the party in the first place. I also thought it was pretty clear, from the extremely cautious way he first addresses her at dinner, that that was the first time he attempted to talk to her about this at all. Given their previous mutual history - that she didn't want to have him, that she almost torched him and herself in her sleep - it's easy to infer that, post-accident, the resentment must have been so strong that they wouldn't have been on speaking terms at all. As far as this part goes, the movie gives you everything you need to work with.