We started early in my family.
At 5 you got a knife and BB gun.
If you could handle those, at 6 you got a .22 rifle.
My first was ye olde slipjoint Boy Scout knife. I never cut myself with it (I was so in awe of being allowed to have it, I was supremely careful. It was only when I was older and sure I knew everything about knife handling that I started cutting myself).
Tyhe progression where I grew up was you first got a slipjoint: BSA knife, or SAK. You were one of the boys when you "graduated" to a Buck 110. We wore those leather sheaths with real pride. After that it was usually a leather-handle Estwing hatchet. When you started hunting "for real" (meaning deer with a shotgun around age 13), you got a fixed blade, usually either a Buck Vanguard or Ka-Bar USMC.
I would say that what to get should be something that you have already that you could give, that way it will have a bit more meaning. If you don't have something now- buy something and let your child see you working with it. Use "Dad as a hero" to instill more trust and honor into the first knife. IMO only.
Now THAT is excellent advice. We would always be "Jonesing" for whatever our dads had. I wanted a Vanguard so bad I could taste it because my dad had it and my dad only had the best. I think I even remember a fistfight with my best friend at the time over which was better: Vanguard or Ka-Bar. (Side note: I think it's funny these days with kids cutting/stabbing/shooting each other. We were always able to resolve our problems with a good fistfight, never thinking to pull out the knive that were on our belts, or shoot each other with the .22s or later shotguns.)
If you're going to give him something at 5 (if he's mature enough it's no problem, like I said, that's when I got started out), spend the next few months showing him how to use knives for various things, and let him use yours (again good idea if it's the same model as you're getting him. Even if you have to buy two, one for you to use and one to give him. I liked my dad's stuff, but was always happy to have a brand new one, that way, all its history was mine.). Then when you give him his, he'll already have the basics down, and will feel like it's not just a gift, but something he's earned the right to have.
As for what to get, I really think a Buck 109 (smaller than the 110) would be good. If you want to give a fixed blade, I like the Jarvenpaa knives better than Moras, and specifically this model: Utility puukko with safety grip
http://www.ragweedforge.com/FinnishKnifeCatalog.html
Good size, has the bump to keep from slipping over the blade, and a neat looking sheath.
Oh, don't forget to get him a sharpening stone he can use, and show him how to use it.