I've been in your son's shoes. One really important thing you have to know is, the more importance you give to a stutter....the stronger it becomes. The more you see it as an obstacle, the more it can get in your way. Never reach a perspective where you see your son as someone with a disability and NEVER put that idea into his head......EVER. That's the worst possible thing you can do. Stuttering is just a quirk. I've been stuttering from age 5 and throughout my whole life. It's never really stopped me from talking that much. Some time during my late teens, I became very self conscious of myself and all my quirks. That's when I stopped talking and all my problems began. I think this might be what's happening to him. He's at the age when people become self conscious about themselves anyways. I think no one mentions it at his school because he keeps to himself and doesn't bother with talking. And from the looks of it, that avoidance behavior you mentioned (about him not talking to cashiers) is only going to increase and its going to cause him problems later in life. The best thing you can do now is to simply encourage him to talk his heart out.........no matter how the words come out. Don't talk about his stutter or put the idea into his head that its a problem. Just ensure that he's talking and getting messages across to people. Stuttering is a form of performance anxiety. Anxiety manifests in different ways in different people. The more talking practice he gets, the easier its going to become for him. And its important for him to know that its perfectly fine to stutter when talking. But he has to keep at it and not fall into the pit of avoidance.