Bald patch

My 18 week old pup has a balding patch between his eye and ear. When I first got him at 8 weeks his fur where his ears meet his head was thinning, this now seems to be growing back. He does scratch the balding patch, but not excessively so, and I've only noticed as I'm aware of the patch. I use Advocate which should take care of mites and the vets just tell me to keep an eye on it. In fact, when I mentioned it again yesterday when he was microchipped, I came out with some ear drops! His ears are slightly waxy, not enough to necessitate drops, and the vet himself advised they are not connected to the bald patch .

Any advice please or past experience of this? He is a 3/4 Dachshund 1/4 JRT.

Thanks