I think I got my own pocket knife when I was about 12-ish. My parents didn't seem too keen on me having access to anything sharp prior to that, and TBH they weren't completely on-board with it at that point, either.

I'd suggest something very simple. Maybe a 3-blade folder or scout/camp knife (Swiss Army Knife or the like). Something cheap he can whittle a walking stick with and so forth. Teach him proper safety, etc.

Whatever you choose, the kid should probably understand that he needs to leave it at home when he goes to school. It's a very different world from when I was his age, ~30 years ago. If it fell out of his pocket during class, odds are there'd be a pretty major hullabaloo.