Reporting
Depending on what type of

Depending on what type of flies these are, they may be biting flies or flies that are attracted to sores, feces/urine/saliva, matted fur, etc. Check to make sure that he does not have any wounds that might be attracting them; these would require immediate treatment.

If no sores/wounds are found: Older cats may not groom themselves as often. Therefore, there might be something on the coat that is attracting the flies. You might try bathing the cat (be sure to dry him thoroughly, for his health) to clean him of any possible material that might be attracting the flies. Ask a vet as to the best way to go about it (use lukewarm water), or have a pet groomer bathe him. He might also be visiting an area that flies like, and that plant residue or other material might be getting on his coat.

In case you haven't seen it, this Web site discusses various fly problems of animals:

http://www.merckmanuals.com/pethealth/cat_disorders_and_diseases/skin_disorders_of_cats/flies_and_mosquitoes_of_cats.html

Hope this helps!

Best wishes,

The Old Farmer's Almanac team