Firstly I recommend you take your cat to the vet. It's important to determine if your cat has some sort of parasite or a fungal infection. The vet may also take a blood test to rule out any diet issues. If this isn't the problem, it can be a case of overgrooming.
Overgrooming is a stress related behaviour - and it is treatable. But the treatment lies in the discretion of your veterinarian. Untill you take it to the vet, try to distract the cat with toys (NOT treats), so it can temporarily forget his compulsive behavoir.
Best of luck to you and your kitty.