16 year Old cat won't clean his furr! ??

I have a 16 year old cat whom I've adored for as many years. For the past month or so he hasn't been grooming himself and has gotten really nasty and dirty looking. He is an outside cat only because he wants to be. We've tried to get him to come in but he refuses.
His sight and hearing are impaired his eating habits are good. Last night I took some scissors and trimmed alot of mats out of his hair out of despiration. He just lay there and let me do it and didn't move. I think he actually liked it. I want to bathe him but fear him scratching me or worse yet something happening to him since he is so old and all. If anyone could tell me what to do I would be greatful. And also why he won't clean himself.