A trainer is suggesting a thunder shirt. That is why I asked.
A good trainer won't be slapping a gimmick on a fearful pup - that's why I said a good one.
If you need to, consider a behavioralist.
There is a difference between actually monitoring a pup's thresholds and working on expanding those versus just plugging treats in the presence of something they fear. I run into way too many trainers who call plugging treats "counter conditioning". It's not.
If the fear is so intense that you are unable to work under the pup's threshold with a skilled trainer, a behavioralist may prescribe a temporary medication to help relax him and artificially expand his threshold enough for you to build it up. Then he can be weaned off the medication when he has built a higher threshold for the stimulus that is causing the fear reaction. Of course, that is worst case training scenario. Hopefully a skilled trainer or behavioralist will be able to teach you how to work under threshold and build the pup's tolerance.