Prolonged thumb sucking in a child over 5 or 6 is most likely created by the anxiety felt from the parents worrying too much about the child sucking his thumb. Experts say that an older child still doing it has been locked in to using it as a mechanism for self-calming, which is not a bad thing. It may hint at the fact that he has some socialization issues and may need to learn how to play more with other children his age; he needs more playtime with others. 
You should consult your pediatrician, but speaking to you as a behavior specialist, I don’t believe that this requires any discipline. I think you should just ignore it and let his peers and the outside world let him know (without your involvement) when it is time to put down his thumb.
My youngest daughter sucked her thumb later than we wanted her to but we ignored it and she stopped on her own.
I’m not sure what exactly did it, but she decided when to stop. There is a level awareness that they develop when the time is right and so do we. Even as adults we have these moments where we suddenly become aware of something we are doing (or haven’t been doing and should) and think to ourselves “oh my gosh, how come I didn’t realize this sooner?” It is similar to each of my children (and you will experience this as well… so be ready) going through the phase of puberty and needing to use deodorant. I noticed it right and away, bought it for them and then we showed them how to use it. Getting them to use it regularly was a different story. I finally realized that I had to let go and not worry that they were going to smell like B.O. all their lives. All it took was a couple of their peers to tell them they smelled and they started using it regularly.
So here are my headlines for this topic:
- Let go of things that are not critical or you’ll create a new disorder in your child
- There is a time for every season and every season will arrive naturally in it’s own time
- Your child will be more likely to open up to you more often by remaining calm and avoiding being anxious
Bill Corbett is the author of the book “Love, Limits, & Lessons” in English and in Spanish, and the founder and president of Cooperative Kids. He has three grown children, two grandchildren, and lives with his wife Elizabeth near Hartford, Connecticut. You can visit his Web site www.CooperativeKids.com for further information and parenting advice.