Hi K.,
I'm the mother of a 2yo thumb sucker and thought I would offer some encouragement. When my son began to find comfort in his thumb, I noticed that everyone around me was more concerned with it than I... even complete strangers! You may know the comments, "when ya gonna make him stop"... "don't you know that's bad for the teeth, etc." I started to do a little research on the best methods, harm, reasons why, etc. The stuff I found was profound for me. In a research on adult thumb sucking the ONLY correlation across the board was that at some point they were forced in some way to stop. In other words, attempting to stop the comforting behavior almost made it more of an issue; they would hide it, want it more, etc.
The best advice I read was to wait. As you can read from other posts, kids will take notice in grade school that their peers are not thumb sucking in public. It may then become something that they WANT to stop and they'll be old enough to communicate their desires with you. Here is where we can play a huge part by offering our loving assistance. Decide together on code words or signals like a rub on the back if you see them begin to thumb suck in public. Talk about ways to reduce stress at school or alternative ways to cope with a stressful moment. I've also read allowing the child to thumb suck at home or in the privacy of their room at night when they're unwinding. No different than our bubble bath, glass of wine, or quiet book... they need a way to comfort and relax, too. Ideally, the thumb sucking will ease when they mature and learn new ways to relax.
Most importantly, just be on his side. We'll love them no matter what, so make sure he knows that! Best of luck!