Non-nutritive sucking (any sucking behavior that is done habitually or during stressful situations as a coping mechanism) is a very common problem. Whether it be the pacifier, the thumb, or anything else, it can have numerous effects on the development of the facial anatomy, the incidence of ear infections, and the spread of other disease as well.
Furthermore, it is a social problem as well, leading to teasing at school and on the playground, as your son has no doubt already learned. While there has been much debate about the correct age to intervene, it is generally agreed that it is appropriate to begin to seek the help of a medical professional at about the age of 3, and there is little debate about beginning therapy at a point before the permanent teeth have come in.
Given that your son is 8 already, he needs to speak with his pediatrician
or pediatric dentist
soon. There are many methods that can be used to help, including the temporary placement of a device that makes it difficult to suck the thumb. Please speak to your pediatrician soon about helping your son to stop this habit.