I was desperate for my son to stop sucking his thumb as I did it until I was 9 and then had to endure 2 years of train tracks. His thumb sucking reached peak levels a month ago when he had tonsillitis, refused to eat and just plugged in his thumb. His front teeth started to stick out - my big fear. I was very pessimistic about him ever stopping because I never did until it was too late.
Initially I tried putting a plaster on his thumb when he was awake and he literally FREAKED OUT. I think it was the loss of the thumb and the fear of a plaster which he has never had before.
So, here is what happened. He came to a physio appointment with me and the physio strapped my back up with thick elastoplast. My son and I talked about it and he understood that my back hurt and plasters would make it better. I talked to him about his thumb hurting if he sucked it and I would have to put a plaster on it.
That night, when he was asleep, I stuck a Shrek plaster on his thumb - very firmly (I didn't want him to swallow it if he did suck his thumb). In the morning he was holding his thumb out and saying that a plaster was making it better. He kept it on all day. I took it off for his bath, left it off until he was asleep again that night and went in and put a plaster back on.
Since then, touch wood, he has broken the habit and not sucked his thumb since. He went from sucking it pretty much all the time, to never, in 2 days flat. I am so relieved.
Good luck if you try this! Just make sure those plasters are stuck on tight.