Is there a reason why he needs to have his collar on all the time? If you could do away with it for maybe a week and see if he's still scratching, that would really help.
I don't put collars on either of my dogs when we're indoors. Lyra doesn't even have a collar--I use a step in harness for her whenever we go on walks. But other than that, when we're inside or in the fenced yard, neither of my dogs wear collars or harnesses. But I'm also VERY vigilant, so I know for a fact that neither of them can ever get out of the house/yard on their own.
Cadence the tricolour Papillon (DOB: July 2nd, 2009) & Lyra the blenheim Cavalier (DOB: April 28th, 2011)