Well, you could try putting the thumb from a small neoprene rubber glove over his thumb, and letting him suck that way, but that means ruining a pair of gloves. Do not use latex gloves for this. Some people are allergic to latex, or can become allergic to it later on. Another idea would be to use a pacifier. You could keep it pinned onto his shirts with a cord or ribbon, so he wouldn't drop or loose it. Discuss this in a nice way with him. I think at two he is old enough to understand why he should give his thumb a holiday, and use the pacifier instead.
I would not worry about him wanting to suck something at his age, or trying to get him to stop against his will. Most kids will stop on their own eventually. I stopped when I was about 5 and was in kindergarten. I had already cut down a lot, but once I was in kindergarten my feeling was that "big kids don't suck their thumbs", so I stopped doing it at school, and eventually quit entirely.--Besides, I didn't want to give the other kids any more reasons to pick on me. They already had more than enough.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau