When you tense you articulatory muscles voluntarily when you are by yourself, there is a 99% chance that you will not stammer. because a stammerer will not stammer when he is talking to himself. So,the problem arises when he is talking to someone else, he faces fear while talking to other person , then he faces fear which ultimately or involuntarily tenses his articulatory muscles , which makes him unable to control the tensed muscles nor be able to talk.