Don’t work yourself into a state over this, stuttering can happen at anytime in their life, in my case when I was around 6 or 7 I ruptured my spleen and the shock caused me to stutter after, I am now 19 and barley stutter. I understand it can be genetic but there is a lot more to life then worrying about this, you are in the best possible scenario, if your son does develop a stutter, he will have a great role model to help him along the way.