If a person is going to get into modeling, even at a young age, they need a real knife that cuts easily and cleanly. I suggest one of the contoured X-acto #1 handles with a #11 blade. For an eleven year old there should be rules of its use, only when an adult is present to supervise. As skills and confidence grow, he could use it with just an adult in the house. Teach some basic first aid skills, as cuts will happen, which is not bad, as cuts teach valuable lessons. I can still look at my hands and see scars from 'lessons' I learned 40 years ago, and they all made me a better modeler.
In addition to a good knife, I would suggest these items.
1) A metal rule
2) Miniature files, especially a pointed round, half-round with wide flat, and a square one. You will accomplish 90% of your filing needs with those 3.
3) A six inch metal file
4) A pin vise with a good selection of drill bits.
5)Tweezers. Nice pointy ones. I use my curved one the most.
6) Razor saw w/miter box. I had used X-acto's saws for years, and was constantly bending them. I switched to Zona saws about 15 years ago, and have been happy ever since.
7) Marking devices--- a scribe, a mechanical pencil, and some Sharpies.
8) Cutters. We all start off with small electronic diagonal cutters, but sooner or later you will need some flush-cut pliers. I have one for plastic sprue, and one for metal. (The one for metal will work just fine on plastic, it just doesn't get into as tight of spaces.)
9) Needle-nose type pliers. Smooth jaw is preferable for modeling.
10) Last, but not least, a roll of paper towels. Don't think I could function without a roll of Bounty Select-a-Size handy.
Of course there are others, more specialized, but this is a good beginning. No need to buy them all a once either. It may take several years for a newcomer to require all of these.
I'd add metal straight edge/ruler and a pair of trianglar squares. Maybe a T-square for fun.
That ruler will certainly keep fingers out of the way, and might prevent the straight edge from shifting as you draw a blade along it. I've never seen a metal rule like that, but I'd certainly like to try one.