Honestly, as a former thumb sucker, there's absolutely NOTHING you can do about it if he doesn't want to stop. And I was QUITE a bit older than 12 when I stopped. There's a girl in our son's Kindergarten class who sucks her thumb. I don't think the other kids actually notice.
If he stops sucking his thumb, that won't shield him from other kids forever. One of the things about sending kids away from home is that you lose control over who they interact with. Not all of his interactions will be pleasant, but most of them will be fine. Are you perhaps anxious about his going to pre-K and focusing your attention on this?
The kids in our daycare can do the soap just fine starting at 12-18 months. Really. I'm assuming the teachers don't just abandon them to the bathroom? He will LEARN to do the soap.
Please, for you son's mental health QUIT telling him that he's not a baby any more and should quit for that reason. It's not working and it might make him quite a bit more anxious about growing up. Comparing him to mom and dad won't help either, because he can clearly see that you are much bigger, stronger and wiser than he is.
If you feel the need to talk about it, then tell him the truth: You're worried that he'll get sick from germs that might be on his hands, and that as he gets older, sucking his thumb might make his teeth crooked. But, at his age, don't expect this to do any good. Our dd will scratch her bottom with one hand, wipe it off with the other and then tell me that she's gotten rid of the germs! "I don't need to wash my hand mom, I got my germs off with the other hand!" Perfect toddler logic. If you can't see them, why not just wipe them off with your clean hand?
You're better off giving him things to do with his hands so that he isn't able to suck his thumb. I wouldn't actually introduce a pacifier, as that picks up just as many (if not more) germs, isn't any better for his teeth as he gets older, and he can use it while doing things with his hands. At least with thumb sucking if you want to play or build, you've got to take your thumb out of your mouth.