What are you feeding him?

The cat needs to be groomed regularly with a medium-toothed metal
grooming comb.  If the matts are bad, take him to the vet and have him
shaved - then start over. The dry skin under the matts is the beginning
of a skin sore.  If you leave the matts on too long, the skin will
become raw, open and become abscessed.  He needs the matts removed and
to be bathed in a soothing shampoo such as ecto-soothe.  Once the matts
are off, if he is long-haired, daily combing is a must, short-haired
cats need grooming weekly, at least.

-L.

I have this same problem. My Charity has extremely long hair and its
hard for her to keep up with it on her own. Unfortunely she has a mean
streak in her and she rarely lets me brush her. I have noticed that
the top layer is fine buts its underneath where she and I can't groom.
I can't because she won't let me for any lenth of time suitable to
comb them out. Sometimes I just cut them off as much as possible. I
keep warning her that if she doesn't let me brush her properly I am
gonna take her to a groomer and shave her but she doesn't listen. LOL