Yes you need to brush him.
Keep a brush by the couch where you
can easily reach it when he hops in your lap,
and give him a five minute brushing daily.

Shampooing will only make them worse, and
the cat is not likely to care for it either.
Get some rounded scissors and cut the matts out.
Or, take him into the vet, or a cat/dog groomer,
hold onto him, and let them shave that area.
Only takes minutes.

The thing to remember is that matts are very
uncomfortable for the cat.

If you are brushing daily, you will notice
when a matt begins, and can snip it right at
the start. Don't try tugging it undone with
brush or comb, that just hurts the cat, and
makes them fear brushing etc instead of enjoying it.

Are you feeding the cat a high quality food?
Unusual for a short haired cat to matt.

Are you sure he's a short hair?? Haven't met a short haired cat with mats
yet.

Whatever his fur type - regular brushing will help. Treat your cat as you
would yourself - and here's hoping you brush your hair and teeth regularly.

i have a longhair.  the only way which I have found to prevent mats is
to comb daily and more than once if possible.  I keep combs in
strategic spots around the house, near his favorit perches, that I can
take a swipe at him often.
I use the comb with metal, rotating teeth and draw if thru the coat
verrrryy slowly.
good luck.
val

I have a short haired cat who gets matted around her lower back and
haunches.  It is just the underfur that gets matted and it's generally
too close the skin for me to want to try to cut.  And she gets
downright mean and cranky when they get too bad.  She hates to be
brushed (my other two love it), although I still try to do it at least
semi-regularly.

I knew the mats were uncomfortable for her - hot, itchy, irritated
skin underneath - so finally took her in to my vet to have the mats
shaved off.  To my suprise, they told me they could brush out the mats
if I would agree to have her lightly sedated.  I just wanted my girl
to be happy again, so I agreed.  She was MUCH happier and MUCH more
like her playful, loving self after I had that done.

I've had to do it a few times now, but worth the expense to me.  I
thought she might be too fat to reach her backside (how embarrassing!)
but my vet says she just starts with one tangle and it grows and
starts other tangles and finally she just gets overwhelmed and stops
grooming there.

I did check with our local PetsMart and with a local groomer about
having the mats shaved.  They both agreed to set an appointment time
and let me just wait there while she was shaved, no having to leave
her there.  I asked about buying a shaver and trying it myself, but
EVERYONE told me that it's hard to shave a cat if you've had no
experience, as the skin is very thin and the mats tend to form close
to the skin.  So, I never did try either at-home or professional
shaving, but it might be an option for some people.