Was his hand actually on the trigger, is that how it fired? He pulled the trigger randomly, not thinking there'd be a bullet in it? Or did it just go off in an unexpected manner without any touch to the trigger?
Either way, in this case, he was negligent to not realize there was a bullet in the gun, ESPECIALLY if he (accidentally?) pulled the trigger while the gun was aimed at his son.
Guns don't kill people... people kill people by using guns.