Not sure from your post how old the pup is, but it sounds as if he is generally fine and it is just the traffic which is the problem.
I would tackle the problem this way. First of all, take him (drive him if necessary, that's fine) to somewhere where you can control the traffic he sees. Maybe a quiet road where lorries don't go, or a supermarket car park near the back where it is fairly quiet.
Make sure he is hungry - you say he doesn't like the treats. This is either because he wasnt hungry, the treats were not very interesting, or he was too scared to eat them because the traffic was too distracting and scary

If a dog is very scared it is not a good idea to force him to face his fears, as that can make it ten times worse. You have to show him moving traffic is fine.
Ideally, use his everyday kibble/dry food (wet food is more difficult for obvious reasons!) and ONLY feed him near cars. This is "classical conditioning" is all about association. YOu won't be rewarding him for his fear. If he only ever gets any food near cars, he will eventually start to see cars as signalling something very pleasant happening.
Food is a primary reinforcer for dogs, i. e. it is necessary for life, so it can be used very well in this rather devious way ;)
At first feed him when moving cars are at a distance, and gradually start taking him out towards more busy and noisy areas....but dont be tempted to move out too quickly. Take it steady. Don't try to calm him if he shows fear - either use a jolly hockey sticks voice, or ignore the fear reaction totally, as some owner's soothing can seem like a reward for the fear reaction.
Hth a bit
Lindsay