From my experience, you should talk to your son about stuttering and have a constant open dialogue about it. My grandma stuttered too and we never spoke about it. It was always an unspoken bond we shared. I wish that she did bring it up and we could have shared our experiences together, but we both feared bringing it up out of awkwardness. I say talk to your son, share in the experience of it, and make an open conversation about stuttering. It will help him accept it and understand it