I do not think this is fleas from what you are describing: 1) none of the other dogs are having problems, 2) you and the groomer are not seeing fleas and 3) you used Revolution which is a wonderful medication.
Unfortunately that means we are looking at another cause of itching and as you had mentioned allergies becomes high on the list as always.
How long have you had him? Any patches of hair loss? Any redness? Any other clinical signs?