Usually cats are self-cleaning. And if he doesn't stink or have matted fur then you are doing everything right.

I've only bathed one cat before and that was because we had a flea infestation and he needed to be bathed.
My previous neighbor gave his cats regular baths, I think once a month. But they were used to it and tolerated it.
A 15 year old kitty who has never had a bath...................if you attempt this, get lots of band-aids!!!