Serious question here. How could he be liable if the gun is left in his truck concealed at a shop with no further input from him? Maybe something hitting the trigger while the truck is moved is all I can think of.
How do you see him culpable if the pistol is stolen from his truck? He would not be liable if someone steals his handgun from his vehicle and uses it in a crime no worse than if someone steals it from his home and does the same...
If a child/juvenile obtains access to it, you can be held criminally liable. if the gun is used in a crime, you are always at risk of civil liability.