Make him as happy as you can. Pet him lots, give him treats. There was a cat we had, that was older than me when he died of throat cancer at 17, and it was a very sobering experience. As I said, do your best to make him happy, pet him, tell him you love him, and give him treats. That's the best I can suggest, and I hope it's not to hard on you.
edit: That sounded a little wrong, what I meant was, I hope you'll be alright when the inevitable happens, losing a pet can be a very hard experince, I lost a gerbil I was very attatched to last year, and even though gerbils arn't "as much a part of the family" as cats are, it was a very hard time. Just keep in mind to remember all the wonderful times you had with him, and not dwell on his death when it happens.