TeamViewer is easy but requires the other party to tell you their ID and password for the session.
I don't know how savvy your Grandpa is but I get frustrated easily when dealing with people that say, "I don't have a password"... or "I don't have a blue box with a white circle and two blue arrows going left and right" or simply have difficulty relaying that simple information even though you've shown them where the box is etc etc.
I would get a free LogMeIn account, install it on his computer, give it a password that you know, and then can log in whenever you want, without him having to activate anything.