I haven't dealt with this specific issue in a 10 year old, but what I've learned about behavioral intervention would tell me a couple of things.
First, identify what purpose the behavior is serving: Is it a way to cope with stress? Does it feel good? Is it a way to get attention?
Second, instead of trying to eliminate/punish the behavior, focus on teaching a replacement behavior as an alternative that will meet the same need that is met now by thumbsucking. I couldn't say what that would be without knowing the purpose of the behavior, but for example if it is a stress release, maybe he could try squeezing a stress ball or something similar.
This link explains the process in much more detail: http://dcps.dc.gov/DCPS/Files/downlo...t-Behavior.pdf
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