If your dog is reacting then the sound is too much.
for desensitization to work your dog needs to be relaxed at every stage of the process. IT TAKES TIME.
if you carry on exposing your dog to levels of sound which he finds stressful there is no wonder he is worse!
Who gave you the advice? It is just not the way to do it for a dog that is already reactive to loud sound.
It has taken me two years of work to desensitize a noise phobic dog to just a vaccum cleaner!