I didn't think it could possibly be fleas, either, and they are impossible to find in that dense GSD fur.
Get a flea comb. I ran it at the base of his tail and found the offending flea in one minute.mi one is found, there are surely more.
If it is fleas, the whole house will have to be treated, along with the dog.
I have found that diatomaceous earth applied at the base of his tail after a good medicinal shampoo and thorough drying have stopped the itching.
DE can also be dusted in the carpets and around the house to get rid of the insects.
Just use care not to inhale it.