The O. Niger Cat will grow quite quickly. I hope you have the room for him. My O. Niger cat is over 30" long. It has a larger diameter than a 2 liter soda bottle. However, a very docile and entertaining member of the tank. I got him 2 years ago when he was slightly smaller than yours. Enjoy!
I like how every time someone posts up about there cats someone always has to start in about housing them and how big they get...if you have a real question about something though you'll have 50 views and no posts..lol