The investigation is still going, and it's completely possible that the LE are not going to share full details with the public so they don't give the potential suspects information on what they know or don't know.

The likely scenario is that the suspect rammed Cali's car, Cali comes out running and wondering what just happened, the kid was probably apologizing and letting Cali know that it was an accident, while Frank was more concerned about not attracting attention. So he probably just said not to worry about it and they shook hands. Then the kid took out the gun and shot him.

I'm still having a hard time believing that this kid would do something like this randomly on his own. There are other ways to deal with stuff like this. If he did act on his own, then he must have severe depression and is on some sort of medication, because no sane person goes from having a clean record and an honest job to killing one of the major bosses of one of the big families, that's insanity. It would make more sense if he was doing this as a favor for someone to move up the ranks or something, but on his own? Still not buying it.