Because of the location, I'd first suspect it's from his scratching, which could be due to ear mite irritation. (look in that ear: is it really 'dirty' looking? then ear mites, or even just filthy ears, may be the culprit).
Step 1 would be cleaning the ears: you can use human ear-cleaning fluid if you don't have pet ear cleaning stuff. Either squeeze some into the ear and rub outside at the base of the ear to loosen debris, and then let the cat shake his/her head, then wipe out any good from the outer ear with a cotton ball, or squeeze some on a cotton ball and gently wipe the inside of the ear, being careful not to push anything downward or poke down into the ear (think of gently wiping any gook up and out). Do NOT use a Qtip (too easy to puncture something).
For either task, it's easiest to sit the cat on the floor, then straddle him (legs holding him in on both sides, without you actually sitting 'on' him) to keep him controlled. When he shakes his head, be ready for lots of gook to fly (if his ears are really dirty), so maybe don't do this in a white carpeted room with fancy upholstery, LOL!)
Another possibility (which is what my cat ended up having when the vet thought he might be carrying ringworm) is some kind of allergies that make him/her itchy and scratchy. It could also be dry skin that makes him/her itchy. If your house is really dry (is YOUR skin dry? especially on the legs?), you might want to get a humidifier.
Definitely take him to the vet to have it checked out. As others have said, it could be something as innocuous as having gotten in a tussle with another cat or having dry skin...or as potentially serious as mange or ringworm.
