Thumb sucking becomes a problem at any age when it interferes with normal developmental achievement and social interactions. It is often viewed as immature and socially undesirable which may affect their relationships with others. A child who sucks his thumb may be ridiculed or teased, which can negatively impact his self-esteem. As you point out, dental problems are the most common adverse consequence of thumb sucking, particularly among children who suck persistently day and night.
Primary goals of treatment are to prevent dental complications and maintain your child's self-confidence. A child should be involved and interested in stopping. Parents need to remain patient and understanding, offering emotional support without criticism. If your child's teeth are already "ruined," I would definitely recommend contacting a pediatric dentist to evaluate his teeth and to see if an intra-oral appliance to interfere with the thumb placement would be helpful.
You could also give simple reminders, rewards, and frequent praise when your child is not thumb sucking. Try to change this habit from a pleasurable experience. Gloves or socks on the hand overnight can serve as reminders. A bitter tasting substance, available over-the-counter, can also be applied each time a child is noticed to be thumb sucking. If these behavior modification and positive reinforcement techniques aren't working, his pediatrician may have other suggestions.