It could be either that he is stressed and overgrooming or possibly he may have a bit of an allergy or dermatitis. Is he scratching or grooming more than usual? Does he seem ok in himself behaviour-wise?
Last year my cat Cash developed bald patches on his back legs, wrists and also on his tummy and ears. He looked awful. There were a few fleas around at the time so I totally blitzed the house with Indorex and treated the cats over several months with Effipro, Frontline & Fipriline. The fleas stood no chance & died out and Cash grew his hair back! (I also fed him Royal Canin skin & hair kibble and popped some evening primrose and salmon oil on it too, only a tiny bit though). He now has the most lustrous gorgeous coat ever so I reckon in his case it must have been a flea allergy.
Sometimes you don't notice fleas but even indoor cats can get them as the blasted eggs lay dormant for ages apparently.
