I, for one, always consider The Longest Journey on a par with the Syberia Games.
I actually like TLJ and its sequels quite a bit more.
(Not that I don't consider the Syberias to be excellent games as well.
) The language and adult themes may make them less kid friendly, however. It depends on your grandson and what you think you, he and his parent(s) are comfortable with.
I also think Ana's suggestion of Lost Horizon is a really good one. It has traditional adventure game play like the Syberias, a fun adventurous story, and I don't think the puzzles are that difficult at all.
A game no one has mentioned, and one I think would be perfect for an eight year old child is the new King's Quest. The first two chapters are great, long and mini games by themselves. They are also pretty self contained, so you don't have to worry too much about the waiting time between episodes. The first episode has some QTE's, but they (luckily) did away with those in episode 2. It's available here, if you're interested and don't have it yet -
http://store.steampowered.com/app/345390/?snr=1_5_9__300.Another fantastic one might be The Night of the Rabbit. It has a young protagonist, beautiful 2D graphics and a really nice, charming story. It's available here
http://www.gog.com/game/the_night_of_the_rabbit and here
http://store.steampowered.com/app/230820/. It does have some pretty challenging puzzles, though, so you may want to keep that in mind if it's an issue.
Also, The Book of Unwritten Tales games might be perfect as well. I assume you may have them already, but I can give you links to those, too, if you'd like.