My male cat had a bout with hair loss/hair pulling. After spending $200+ at the vets this spring, running all sorts of tests and have them tell me they couldn't find anything wrong, I figured out it was stress related because I hadn't been home and given him enough attention. Since then, I've spent more time with him and he's stopped doing it.
In fact, he's laying on my foot now and purring up a storm.
Cats, in spite of their reputation, can be very sensitive creatures. Any change in my cats' routine is met with stiff opposition and bad reactions. Maybe you can look and see if anything has changed in their environment that might cause him undo stress. Just a thought and good luck with your kitty.