I have a 5 yr old male cat. He has lost alot of fur on his back end and stomach area.?

His gonads are still furry. I did take him to the vet. They chalked it up to fleas but I dont think that is it. I give him flea meds. They did do bloodwork and found out he has heartworms. I have since switched him to Advantage multi. Today I notice an additional huge patch missing on his upper thigh/rear area. He is an indoor/outdoor cat and does not use a litter box. I feed him the same food for a long time. He is a very laid back cat and does not seem to be upset at all. His personality is very lovey. Does anyone have any ideas? I do not see that the heartworms has this side effect. Also, there are no scabs or scratches. I also have other animals and they are all fine.