I've had a lot of experience with boys in Cub Scouts and in Boy Scouts using knives. As the others have said, there is a wide range in maturity and common sense among boys, but an eight year old CAN be mature enough to learn to use knives.

Here is a link with information on the Cub Scout program that I think could help you teach your son. I'm not saying to join Scouts, though that would be a fine think for a young lad - I'm saying to borrow the ideas.

http://www.geocities.com/~pack215/whittlin-chip.html

http://www.crossroadsbsa.org/pathfinder/ptc/KnifeSafety.pdf