If he only stuttered until he was 8, I dont think he had a true stutter. Many children have speaking difficulties that can last into adolescence. I forgot the stats but like 20% or something of children have some kind of disfluency issues which can carry into adolescence, which normally do resolve themselves. But people who actually stutter don't just outgrow it. Im sure your grandpa thought he just outgrew it and told you likewise. I heard this from a lot of people who were giving me advice to, but it doesn't work like that for people who have a true stutter. Theres probably a lot more going on, neurologically with us than with them.