I'm not sure, never seen it before. My first instinct is to say ear mites. But i it is ear mites, you should be able to rub some of it out with a damp or dry q-tip.

It's possible that there is something under his skin, that we can't see, too.

Alergies are another likelyhood. Food alergies in rodents cause 'hot spots' and the rodent will scratch itself basically to death, generally starting in the ears, on the neck, or on the face. But I've never seen this in ASFs as of yet.
If it is alergies then there is almost no chance that he's going to return to normal. The itching usually becomes an OCD type of habbit, and they keep on scratching it even when it clears up.

You can keep him/her around ans see how things go. It would be interesting to see what it is.