S.H.
My daughter is 5 and in Kindergarten and she still sucks her thumb. I have asked her teacher about it and she says my daughter does not suck her thumb at school. My daughter says that she doesnt want to suck it at school because the other kids will think she is a baby. Your son may give it up on his own when he starts school, or like my daughter only sucks on it at home. I havent tried to force her to stop yet, I figure she will give it up when she is ready. Good luck