Why does my son stutter?

Why does my son stutter? He is currently 18. He has been stuttering since he was 11-13 years old. I dont understand what should I do! Once he goes to a college, he will be meeting new faces and I feel that his stutter might affect him dradtically. I just want my son to have a happy life. I want him to enjoy all relationships that he makes in his future and I love him a lot. I am ready to do anything. We also met a Speech Therapist in out area, but that didnt help. The doctor made him read slowly from a book for a month. What should I do? I dont want my son to stutter. Guide me.