I didn't know he was still in business after so many years.

or

I didn't know he was still around after so many years.

or

I didn't know he was still in the game after so many years.

or

I didn't know he was still active after so many years. Does this sound natural?

I think all of these sound pretty natural, but have very slightly different meanings. The first implies someone owns a business and you thought it may have failed. The second is extremely broad, and "around" in this case could mean either "living here", "working here", or "alive". The third one is extremely informal and I might refrain from using the term "the game" as it has some mild connotation towards illegal activities, or at least towards a very competitive industry or workplace. The fourth is similar to the first, but it to me instead implies that you thought he'd have retired voluntarily by now.