I have 4 kids and theyve done a bit of that. What usually worked for me was to say that it can stretch out your nose and it will get really huge, and everyone will notice, and in our society its considered such a gross habit that other kids wont want to play with him. Sometimes pier pressure helps. My aunt growing up, was an ear, nose and throat doctor, and I always remember her saying that people can tell the nostrils have been stretched and flare way out when people are nose pickers, that really had an impression on me. Also, it just has to be a rule set early as possible, just like rules of not swearing etc. I'll never forget the memory of a childhood classmate hiding behind a book in class and picking his nose incessantly. I could not believe that he thought no one would notice! So you see, its something you have to nip in the bud, but at the same time, certain things, like cleaning out ears etc, just have to be done in privacy in the bathroom. Teach him to blow his nose clean in the bathroom. We all know people have to do it to a certain extent, but no one wants to watch him do it, I sure know people wouldnt come hang out with me if i were picking my nose. You have to make it very clear how people will shun him for it, like the boy in my class who I only remember for that! Goodluck to you!