It's understandable that he is sensitive to cars. He needs gradual desensitization.
Start at whatever level of car intensity that he can handle, meaning, he is still willing to take treats, respond to your call, not shaking, whining, distressed. This may mean you need to start with a parked car that is turned off, then turned on, then a bigger car, then cars moving from a distance (start w/ a low MPH street with light traffic) and so on. Bringing a super mellow, non-reactive dog or two will help a lot. You don't even need to be walking. You could find a nice spot in a park to sit that is far enough away that he is aware of the cars going by but is able to sit or lie down (after a few minutes) and eventually ignore them...because they're boring.
You can also play traffic sounds https://worldreadypets.com/desensitizing-sounds-library for him at home for about an hour per day.
The smell of cars may set him off as well. You could rub a rag on your car's tires and engine and then hide treats in it for him to dig through.
You want to put him in situations where he has a reasonable probability of success but not so easy that it does not challenge him. Encourage voluntary confrontation of the unknown. You'll want to do this every day, if you have that kind of time. Set him up for success with baby steps. If it feels like the progress is painfully slow, remember, any improvement is better than zero.
You were smart to reach out...perhaps a trainer could get you started if you don't feel qualified. He is lucky to have you. Don't lose hope...it's a slow process but he is likely to make progress if you are consistent and patient. Good luck!
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