The plots of KOTOR 1 and 2 might be a little over the head of an average 8 year old. The game mechanics would CERTAINLY be too much for them, unless by chance he's playing D&D right now?
Final Fantasy 7 and 8 have Windows versions, but 8 stinks and 7 has serious technical issues.
You can find several graphics fixes online that not only enchance the visuals, but make the game run better in WinXP with more modern 3D cards.
Lego Star Wars 1 and 2 are both good PC games. They are actually ported console games and not really good RPG's but they are great fun for kids and adults alike.
Septerra Core and Evil Islands are OK too, but damned if I know where you can find copies.
I dont think you will have much luck. PC games are more grown-up in general and RPG's even moreso. If you really want to get such a young kid into RPG's I highly recommend starting with Hero Quest, followed by Descent: Journey into the Dark, and then the D&D Basic Box. Once they get used to dungeon crawling you can introduce them to actual role-playing.
has advice for such things.