How old is he?
Sounds as though he'll get back on track. I second the notion of starting out gently, some place without cars. You really have to rebuild confidence after something like that and it's kinetic as much as anything. No matter how many times you replay the blocked sign and the rest, the trauma rests in the sinew so he should start slowly, but perhaps soon if riding is something he wants to continue doing.
Glad he's okay. Keep posting.