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Your cat almost certainly has an abscessed tooth or possibly a decayed tooth. In cat this age a smell around the head or face is almost always due to a dental problem. Please take him into your vet so they can check this. If it's not taken care of soon, he may stop eating from the pain in his mouth, and he could develope kidney issues also as the bacteria from the mouth will migrate down to the kidneys and cause problems there too.
He may possibly have a yeast infection in the ears; however, they usually don't smell terrible as you are describing. In either event, the vet can diagnose and resolve this for your cat very quickly.
Please let me know if I can help further.