Nighttime thumb-sucking - how to stop!?

[MtnBikeChick]

My 5 year old sucks his thumb at night only. He never sucks it during the day. It is completely subconscious. His teeth are starting to stick out a little and the dentist has advise we stop the thumb sucking asap. However, because he isn't aware he is doing it a reward system won't work. He is very very strong willed, and I think painting the horrid-tasting stuff on his thumb is going to be a nightmare...but perhaps necessary. Does anyone have any experience of stopping night time thumb-sucking?
Thank you

gloves?

sticky plaster on his thumb, or mittens on for bed.

Failing that replace his thumb with a dummy, it doesn't solve the problem but it's softer than the thumb and can be thrown away once the habit of putting his thumb in his mouth is broken.