This is such a hard thing, for any one to feel and/or to give honest advice/opinion about! I always say trust your instincts when it comes to cats, dogs, animals of that close relationship. Usually that is regarding if they are acting different, possibly sick or injured, an infection or something we can't see. Especially cats, they don't really ever show pain, they show the difference that is necessary for them to avoid the pain. In a way, this is similar in the sense that your feelings may be his feelings. Not that he is about to leave you, but that he is getting old, and not the kit he use to be. Don't want to scare you, it's just as an elder family member, you simply don't want to loose them. 16 is a strong age for a cat, you may have a few more years with him! Just be sure to give him the senior discounts!
A year and a half ago I lost Sweety at 16, she was as strong as an ox! Climbing the side of a door frame to the top right up to her last month. I lost her suddenly to congestive heart failure, it all happened in 2 days (thank god for her was only). I was always babying her since she was an elderly girl, then she would climb the door frame and shock me! It was so hard loosing her