13 years old thumb sucking?

Long story short I agreed to take in my neighbors 3 kids because CPS was gonna take them away. They're two boys and a little girl. They're 13, 7 and 3. The 13 year old has a thumb sucking problem. I'm not sure how to get him to stop. My husband just tells him he's too old and to stop and he will for a moment but then will do it again. My sister recommended getting that not bite stuff and putting it on his thumb nail. Are there other ways to help?