
Have you tried to help with the itching with washing in Maleseb? - leave on for about 1/2hr and then wash off
Also camrosa ointment is good and i know someone who has been using Dermacton which has pretty much stopped the itchiness.
How about trying piriton if the vets are happy to see if this assists in alleviating anything
You say all the tests have been done with nothing to say that he is allergic to anything. I would do the following if not already done:
Remove all air fresheners in the house including febreeze
Stop using any shake n vac
Washing all dogs bedding in non bio only
Use only cotton bedding
Check what you wash the floors with, some chemicals can cause irritation
Funny question but probably not related but do you have ants in your garden as these can cause much irritation to a dog - this only came to mind after a friend mentioned this to me
Failing that i would be looking at getting blood tests done at the vets.