Skittish Feral Cat with large matted clump of fur

Any way to help extremely skittish feral cat with a large, painful looking clump of matted fur? Without caging or sedating? He is a very stout persian mix about 12 years old. He has been caught and sterilized as a kitten but won't let any humans near him. Any suggestions? 

Without catching him and possibly drugging him for a grooming session, I can't think of anything