Break the habit if you can now. It's much harder when they're older. Try pinning a sock onto his shirt sleeve so that he does not have access to the hand at all. A long sock that will go up his arm is best and then pin it to the shirt so that his arm and hand are in the sock.
I have a 6 year old that I've been trying to break of the finger sucking habit since she was 2. It's very difficult. The sock method seemed to work best until she learned to just take her arm out of the neck portion of the shirt (she was 3 at the time). It's much easier when they're smaller (which I've been told). I wish I had known that when mine was still a baby.
The bitter stuff for the thumbnail didn't help us either. We've tried just about everything - I even talked to the dentist and she said you can do much more harm if you install a metal bracket in their mouth so that route doesn't work either. Good luck - I think you'll have more success since your little boy is much younger.