You are a very thoughtful colleague! Having been an out of control stutterer until I was about 25 and worked in open plan offices, I can understand his anxiety. Although everyone is different, it is really soul destroying to not be able to say what you want when you want. It is also very difficult for you to approach it as again every is different. Perhaps ask if there is anything you can do to help him - maybe a practice call pretending you are a customer? Until he gets proper useful help it will be a long struggle. The McGuire Programme is a fantastic programme for people who stutter and is run by people who stutter, maybe your company could sponsor him to attend.