Matted fur can be a sign of kidney problems and that the end is in sight. But it could be something trivial that can be cured.
Basically, you are in the horrible position of having to decide whether to annoy him by consulting a vet, well knowing that you may be forced into the decision none of us ever want to take, or to leave well alone, as long as he still seems to be enjoying his life. You didn't say, but I take it he is still eating, drinking, peeing and doing his jobs as normal.
My experience with old cats is that both you and they know when life is no longer fun for them, two of mine the one 16 the other 10 turned overnight into apathetic creatures who were obviously dying. The vet confirmed kidney failure in the one and diabetes in the other, so I saw my painful duty clearly.
Try not to worry too much, just enjoy the remaining time with your old puss.