Scouting usually discourages fixed blades, and sometimes even locking-blade folders at the Cub Scout level.
(BTW - I can see both sides on this issue, and have no desire to debate it)
The quintessential first scout knife is a Swiss Army Knife. Several blades/tools, easy to open/close slipjoint, bright red (hard to lose), and a great knife to be nostalgic about years later when he is a man.
You can't go wrong with any of these:
http://www.swissarmy.com/multitools/Pages/Category.aspx?category=scouting&
They do have one that has the Cub Scout logo on it. If he is a true knife-newbie, then get this one or a Classic SD
I opened the catalog and let my son (an 8-yr-old Cub Scout) choose for himself. He ended up with this one:
My First Victorinox Plus
http://www.swissarmy.com/MultiTools/Pages/Product.aspx?category=everyday&product=54651&
He's not really into whittling and didn't want a pointy tip....but did want a saw...(and, of course, the ever-so-popular toothpick

). So, that's what I got him and he loves it.
If your son is going to be doing some whittling (amongst other scout tasks)...then I would get him one of these:
Adventurer (make sure locking blades are ok with his pack first)
http://www.swissarmy.com/MultiTools/Pages/Product.aspx?category=outdoor&product=53601&
Tinker
http://www.swissarmy.com/MultiTools/Pages/Product.aspx?category=doityourself&product=53101&
Farmer
http://www.swissarmy.com/MultiTools/Pages/Product.aspx?category=doityourself&product=53964&
Also the Cadet, Electrician and Soldier might interest him.
I'd steer away from blades that are too small (under 2") or too big (over 3.5").
I'd also avoid stockman (jack knife) types...as they are easier to lose (smaller diameter and not brightly colored).
And lastly, I'd not overdo it on the blade/tool count if you get a SAK.
Best of luck in your search....be sure to let us know!
Dan