Actually to start desensitizing him to the loud noises - you need to start slowly. If you take him to a place where he will be overwhelmed by the noise, it will only reinforce the fear. It might appear that he's over the fear when in fact he might have just 'shut down'. You need to walk him in a quiet area, but someplace that you can get as close to loud, city noises as you want. As you walk toward the noisier place, watch your dogs body language - when he starts to show signs of tension, stop. Stay at that distance and distract your dog by having his absolute favorite toy there or by giving him high value treats (whichever high value item works best for him). Each day try to take him closer to the high noise area, even if it's just a few more steps.
You could also try walking in your normal places, and watching for trucks that will cause him to be scared. Before the truck ever gets close enough to start the fear reaction, distract the dog (again, with whatever high value item works for him), and praise like crazy if the truck gets past without him reacting. Have him sit with his back to the fearful thing so that he will be paying attention to you. The trick is to pay close attention to the body language so that you stop the reaction before it starts. If the reaction is already underway, it's too late. You can also make a recording of the loud noises and play the recording while at home. Start out with the volume at a level that doesn't cause any reaction in the dog, and slowly start to increase the volume day by day.
One other thing - you might want to have him checked at the vets. There are health issues that can actually cause pain with loud noises.
Livin in a Newfie Drool Zone