Our DS12 has been very sensitive to loud noises since day one. He has seen Illuminations at least 6 or 7 times and though he covers his ears he doesnt seem unusally bothered by the show. He has auditory processing disorder, which seems to cause noise to intesify(at least for him).
The start and end of the show seems the most intense. There are slowdown parts where soft music plays in the middle. We have found standing on the bridge between France and UK affords a very good spot to see the show. Also, this spot has nothing behind it(no buildings) which allows the noise to pass and not reflect. If it becomes unbearable you can duck into the store just below the bridge on the UK side that leads toward BC and BW.
Hope it works out for you. BTW, for the best view, make sure you are somewhere over the water where the canal leads into the WS lagoon. This is where they pull the pontoons and breathless boat tours under to get a great view.