It is a good idea to get him off it as soon as possible, mainly to give his teeth their best chance of developing well. He's unlikely to revert to thumb sucking at this age but rather the dummy than the thumb. You can't throw those away.
I was a rotten mother and made mine go "cold turkey" one night when I'd given them a really busy energetic day and they were so tired they couldn't object for too long. I sat and cried beside them till they went to sleep but after a couple of days they forgot all about it and, best of all, they didn't pinch the baby's out of her mouth.
I like the bribery techniques mentioned but you've got to be firm and make sure he knows that once it's gone, it's gone.
Good luck! When you've done it you'll wonder why you didn't do it sooner!