Scooter
If your son knows the mesechtos that he previously finished, then pick one he knows well and get an English of that masechta (or yiddish if thats his mother tongue and he will understand it better these can be baught at seforim stores) if he doesnt know any masechta that well then do Pirkei Avos as that is something he will find to be interesting. Whatever you choose make sure you get the grandfather a version with a translation. The Artscroll Yad Avraham are excellent.