A.H.
First off, I wanted to mention that I completely understand how you feel. I was panicking when DS was that age,and everyone said "oh, don't worry,he'll stop by the time he's x". Well, fast forward age 5, and he was still sucking his thumb' and yes, had an overbite, but everyone says not to worry until the permanent teeth come in, which they hadn't yet. My ped dentist recommended Mavala Stop. I bought it online and he stopped sucking within 3 days. We used it intermittently for 3 months, before he completely kicked the habit. You can buy it on Amazon. Read the product reviews. It works. That being said, you may want to try it when he's older and has an interest/desire to stop or you'll have to deal with screaming and/or a lot of crying. Good luck!