It sounds like he's frightened, so if he was mine I'd avoid making him walk close to roads at the moment. Can you think of a place that you can take him where you can be at what he considers a safe distance from traffic, and where you can sit or stand with him at that safe distance and let him watch and listen. If he's relaxed, move a bit closer, but do it at his pace so that he can stay calm and gradually come to realise that he doesn't need to worry.