He's at that age...gaining independence but not being able to really decipher what is and what is not a threat. Up your leadership(all family must be on board) with NILIF
and keep on training him. Don't flood him with experiences, if you have to, stay on the fringe of things. When he reacts in the car tell him "OK I've got it, Quiet" so he knows he doesn't have to be in charge of the world.
As he matures he should slowly figure out that everything is not worth barking/reacting at.
Be firm, fair and keep building his confidence in a way that he knows you have his back....it will take some of his stress/anxiety away.