I suggest something that opens with one hand(he can still use two hands if he wants).
You don't want it snapping closed on his little fingers. So that's why I suggest something that doesn't have a snap to it, such as an SAK, etc.
This being said, how much are you looking to spend?
You also want something that's easy for him to close. So a liner lock, frame lock, or no lock may be your best bets.
You also want something small because after all he's just a little guy and anything too big would look like he has a knife made for a giant.
For some things under $40 I would look at:
Spyderco Squeak - No lock so he doesn't need to put his fingers in the way of the blade, but it also won't snap closed on his fingers. A decent steel that will hold its edge well and is easy to sharpen back up.
*Spyderco Ambitious - Another little blade, has a liner lock. Liner locks are easy to operate and you could teach him how in just a few minutes. 8cr13mov steel that holds its edge pretty well for a cheaper steel. Easy for dad to sharpen back up lol. I would take a look at the Blue one. This is actually the knife I would get him. It'll get him used to an easy lock, be in a Blue Color for a Boy, and the one hand opening will be easy for him. He'll like it, I'm sure.
CRKT Pazoda - You can get the large(Pazoda) or small(Pazoda 2) version but for an 8 year old I would go for the small version. It's a nice little knife and he'll think it looks cool. Has a smooth frame lock, again he can be taught how to operate it safely in just a few minutes.
A CRKT Shrimp could be a nice little blade for him too, he can open it two handed but with it having a frame lock it shouldn't snap on his fingers. You can also get this in different colors.
