[anonymous]

Keep him on a short leash, when you see the car/bike/truck approaching make him sit, kneel down with him, make eye contact and say No in a firm voice. Don’yell at him. pat him, try to comfort him.
When he stays calm (after about a hundred times). Praise him profusely. Maybe give him a treat.
He should do it less and less, but if he is startled, all bets are off.

Some dogs do this, other than working with a trainer. Don’t know what to tell you.
Keep him on a leash at all times.
I hope it’s not a deal breaker. :-/