I totally get this. I have a 26 mth old dcb who is exactly the same. I've been working on it since about 18 mths, primarily because everyone was having back to back colds and this wouldn't have helped at all. Honestly, I have seen a significant reduction and primarily because all day long I am constantly saying, "take your fingers out of your mouth" in a firm tone. At first it would have him upset, but now he just stops and skulks off to play.