[cypher]

The fur behind their ears is quite thin anyway so can sometimes appear reddish, it may just be that he's been scent marking a lot since you separated the boys, scent marking & grooming a lot are quite common after a change like this.
Keep an eye on it to see if he does continue to scratch a lot or over groom.
You can't see mites, they all have under skin mites which aren't normally a problem & it's only if their immune system isn't working as well as it should that the mites get out of control & need treatment, this can be due to stress so it is possible the fighting & then the separation may have triggered a problem but at this stage I wouldn't worry too much & just wait to see if he settles.
A small wound is definitely another possibility but if it's only something really minor that should just heal without any treatment.