Do you let him play and be off duty after being in the car?
Animals usually face a lot of scary situations in the wild, but they rarely end up with prolonged stress reactions. Why? They can shake off and run off the adrenaline. It doesn't get "stuck." (Somatic Experiencing therapy for humans is built off this fact.) When a deer runs away from a threat in the wild, once it is safe, you'll see it give a whole body shake. Wolves are the same.
Dog owners and trainers, meaning well, usually try to comfort and hold back a dog after it's faced a fear. When it's a big fear that the dog is having a prolonged reaction to, it's actually better to pause and not hold the dog back, but let it take a moment to get a little space to shake it off.
An example: my dog was afraid of a cow statue in town. I put treats near the statue so the dog would get close. Dog got close and associated statute with treat! Dog was still scared and visibly shaking, but did it for the treat. As we back away, I signaled for her to be off duty... and she shook all over and danced a bit... and then she was totally fine. She processed the fear much quicker when I didn't hold her back from wiggling it out a bit. Next time we went by the statue, she was totally fine. No fear. No reaction. No change in working status.
More complex situation:
At least one a week, we have to walk up a looooong flight of stairs that is nearly see through. She was so scared of them. While we were still working through the fear, training that the staircase is safe, she was given treats and praise going up, and then a moment of off duty time at the top. She could then shake out all the stress. I didn't verbally reassure her once we got to the top. I just gave her space and the off duty command. (Taking off the vest might be the same as a command for other dogs.) She wiggled and moved about, shook all over. and quickly returned to being a very happy dog. We did it a handful of times. She didn't have any lasting fear or associating that working means doing scary things. It took a few times doing this and she was going up and down the stairs without much difficulty.
I'd suggest that the next time you go in the car, do something right afterwards where he can freely move a bit, off duty, shake and burn off the adrenaline and stress... and then return back to duty again. If you try this approach, because this may be new, it may take a few times before he gets it.
But now, my dog knows the off duty command and the shake command. When she faces something scary, she will hang on through the fear following commands just fine, and then knows when I give the command, it's time to let the fear wiggle through her body and shake or dance/run it out a bit, then she can let the fear and stress response go, and it doesn't get "stuck" in her nervous system. It hasn't messed up public access one bit over the long haul. She walks up and down those stairs now without pausing or hesitation, and without having to stop and shake off the fear. She's worked it through.
ETA: if he’s acting depressed when not working... he could be bored. Exercise and brain toys can help.
This all being said, each trainer and each dog is different. You'll find lots of ideas online. Take whats helpful (and safe) and disregard the rest.