It could very well be an allergy to the spray the groomer used. Next time he goes in, ask her to skip the spray and/or change the shampoo used.
Our dogs don't always seem itchy when they have hives, either, and their hives don't look very red. Normally they just look like lumps under the hair and usually disappear as soon as the reaction is brought under control with Benadryl and/or steroids. Might not be a bad idea to ask the vet for the proper dosage of Benadryl for your dog and keep some on hand. Next time the lumps appear, try it, and if it works, the lumps are probably hives... Then you'll just need to pinpoint what it is he's allergic to...
I know....easier said than done, but at least then you'll know it's an allergy you're looking for.