Stuttering can be passed down, but it's not a guarantee. My mother stuttered when she was alive. I stutter, but none of my siblings do. In the same manner, both my parents needed glasses ever since they were in their teens. All of my siblings need glasses. I am in my late 40's now and I still have superior vision. You make it sound like a child with a stutter cannot have a normal life. I can assure you that is not the case at all. It sucks, yeah, but it can be dealt with in thousands of ways. And. who can be of better help to a child with a stutter than a parent that has gone through the same thing? If it truly concerns you as much as you say, to the point that you do not want kids with this guy because of it, then at least be honest with him.