S.B.
My daughter never sucked her thumb until I took her bottle away.
My husband HATED her sucking her thumb. He called her a big baby, threatened to punish her...the more he harped at her, the more she sucked her thrumb. I told him to leave her alone and then, I made a deal with her, I was fine with her sucking her thumb, I told her she didn't have to stop, but no sucking her thumb in public, or even in the car. (I didn't always enforce the car rule especially if we were on load trips and it was obvious she was getting ready to fall asleep).
Telling her she didn't have to stop and telling her the places it was okay worked amazingly well. If she wanted to suck her thumb at night in bed, fine. If we were watching TV in the evenings, she would pull her blanket up over her head and I knew what she was doing under there, but didn't say anything. Before long, she was ONLY doing at night in bed and eventually not at all.
She had a big callus on her favorite thumb too. It did go away.
Another thing you might want to do, which I did and it worked for her....
once she was asleep, I gently took her thumb out of her mouth to give it a rest and air out. She very rarely put it back in after she was sound asleep.
A little reverse psychology might work with your child too....
maybe try taking the focus off of him stopping all together and tell him when it's okay. If he forgets and puts in his thumb in the car, you can say, "We don't suck our thumb in the car, remember? We'll be home in a bit and I know you can wait. That was our deal, right?"
It's worth a try. It worked WAY better than the yucky stuff and the threats and constant bickering by my husband about it.
Best wishes!