Hi Sandi,
The product is called Mavela STOP. It was recommended by our dental office. I don't know if you can find it locally; I purchased it online. One word of caution - if your son is eating foods with his hands (such as sandwiches at lunch time), he may accidentally get the taste in his mouth while eating, too. Teaches better table manners, too, I guess (don't lick your fingers when you eat!).
Here's an advertisement for it on amazon.com.
http://www.amazon.com/Mavala-Stop-Nail-Biting-Sucking-Prevention/dp/B0000YUXI0 Good luck,
Julie