How to get son from sucking his thumb?

You can put something sour or bitter on his thumb. I believe they make actual products for this, I've heard of parents using lemon juice or something similar. When I constantly bit my nails as a kid, my mom bought this gel to put on my nails that was so disgusting it would last even through brushing my tongue. I stopped biting my nails within less than a week. Your pediatrician could probably recommend a product or home remedy.