Most high ranking pro musicians you will find started very young. Get your grandson to learn to read music onto the guitar at some point. I do stress that was at some point. Even if he then ditches it in later life he will have that ability. Being able to read music means you can pitch a song off of the sheet music, it means you can join in with any musician and as a guitar player in later life you can earn some serious cash as a session musician (even if it is a hobby!). It is a running joke amoungst the jazz fraternity I keep that guitar players cannot read music or sight read. I wish that I was better at it. Also make shure he gets a good teacher - that is paramount. He must have fun in the process of learning or else he won't stick with it.
Good luck.