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What you are describing sounds like facial alopecia, which has no cause, but is also not harmful. It usually is more apparent in long hair cats, and is just a thinning of the hair from the ear to the eye. My Maine Coon has it too, and it is perfectly normal. To see a picture and for further information click the link below. the picture to the right is exactly what it sounds like you are referring to.
I hope that helps put your mind at ease.
I like natural foods, such as Natural Balance, but this condition is not known to be caused by diet...so I am sure he is just fine! If you have any concerns, let me know, I would be happy to discuss!