Assuming the parents know about/are addressing this with a doctor or therapist, the best thing you can do for your grandson is listen to him. When he talks, let him talk. Don't interrupt him or try to finish his thought....he wants to tell you but probably can't get it out as fast as he wants. He will notice how well you listen and compassionate you are. Also, do not ask him about the stutter unless he brings it up to you first. Odds are he'll grow out of it as a pre-teens but he will develop anxiety about his stutter if he hasn't already. The more you rush him or ask him about it, the worse the anxiety will be. At 39, I still struggle a bit with my stutter and it's extremely noticable when people are caring and understanding about it. Good on you grandpa for reaching out and asking. Knowing that you're being proactive with this and learning how to support him is so refreshing. He will notice, and it will allow the two of you to build a bond that he will only have with a handful of people throughout his life.