10 isn't that old for a cat. Average life expectancy is about 15 years. If you haven't had him checked by a vet, do. Hair loss like that is more likely to be allergic reaction to flea bites or other parasites or possibly over-grooming due to stress/compulsive behavior.
I wish I could attach a picture of an underboob I saw the other day on the interwebz... probably a good thing that I can't (it was a guy) O_o But holy crap was it funny.