M.B.
Some kids need more oral stimulation than others. If you try to force him to NOT suck his thumb, you could do more damage than sucking his thumb at that age is doing.
My 4 yr old grandson is still sucking his thumb... he will quit when he is ready. We are trying to limit where he does it, though, since his thumb is frequently in his mouth, not just at bedtime and such. (We tell him we can't understand him when he is sucking his thumb, etc....)