Is there a reason you think you should stop him/her from doing it? It's a security and soothing tool that's always there, without you having to find it lost in the crib at night, never have to pick it up off the floor, etc. I wouldn't sweat it, especially at this age.
I was a thumb sucker and despite all the agonizing (including Tabasco sauce - very cruel, because it would even be put on while I slept and I sometimes got it in my eyes) things my dad did, I REALLY needed some kind of security and did it until I was 6 - but not in public. My teeth all grew in fine and straight, and the tooth issue has been proven a fallacy, unless the kid does it pretty much 24/7. I wouldn't worry about it for now. Enjoy the peace and quiet it provides.