My daughter found it soothing to wind down after school by laying in bed under a heavy comforter, lights dim, and chewing on a chew toy her OT recommended. She was so wound up and tired at the same time, she needed a way to calm herself. She would chew on this rubber ring and then usually fall asleep. Sometimes she liked a soothing CD. She wasn't young either when we finally struck the right routine for her- she was 5! We also did a brushing program that her OT taught us. It really helped.
She felt bad at first using the teether, like "Am I a baby?" and we just said, "No, people just need different things to get their bodies calm and this helps you." She only used hers in bed though.
Sounds like it you can find something to replace the thumb, something equally soothing and close by (like, bring it along to school pick-up so he can have it on the way home if he is solo, so he doesn't have to explain it to other kids), and buy 4-5 of them, so you will have them when you need them, that might encourage him to break the thumb habit.
I think we ordered ours from Southpaw Enterprises. She liked the P and Q set and also some grape scented ones. She used them for about one year.
She also LOVES her vibrating toothbrush now. Do you think he might enjoy using something like that orally when he feels the need to suck his thumb? We got ours on sale at Jewel for $4.