No, just let him be. Kids who are not "real thumb suckers" (that is, they didn't discover their own thumbs when they were in the first year of life) don't become serious thumb suckers later on. Your son is just playing "copy cat" as toddlers like to do. They see what the other kids at school are doing and they try it out. He might even think it is funny. If you leave him alone, he's most likely to drop this habit by himself in a very short time.   Sarah Chana