Good points everyone!
Couple of questions:
1. Does your son have any type of empty-hand SD training? (and no, I don't mean sport-type Tae Kwon Do

).
2. Does your son have any type of training for using a knife in SD?
If the answer to either of these ?'s is "no," I would definately consider getting him some instruction rather than just hoping that because he has a knife that he'll be fine. I'm not saying he shouldn't carry one (I've carried one just about every day since I was 6 or 7). Just that to use it effectively requires some training. You wouldn't carry a firearm for defense without knowing how to operate it would you?...same logic applies here.
One thing to remember, especially for someone his age, size, and strength (I'm ASSuming he's your average sized 13 year-old) is that in a violent situation the primary goals are survival and escape, not to "win" as if the situation is a competition. The difference is in the fact that operating under the "survive and escape" mindset your goal is to engage only long enough to create an opportunity to get to someone/someplace safe. In the mall, this might be a police-substation or the security office, or maybe just a store with lots of people. This applies to everyone but particularly to kids. At the studio where I train/teach we tell them that, since they're not going to be a "match" for most attackers in the physical sense their best bet is going to be to run like hell and scream their heads off. If the guy grabs them, hit them with a couple of quick strikes to sensitive areas (groin, eyes, throat, etc.) and
then run like hell and scream their heads off. :biggrin:
ExSoldier brought up a good point with the "expect to get cut" thing. Knives are nasty, in fact, at close range I feel that they're the most dangerous weapon you can encounter. I would rather have someone stick a gun in my face (hasn't happened yet, thankfully) than have to try to disarm someone attacking me with a knife (hope it doesn't happen again). Then there's the fact that knives don't kill instantly like they do in the movies, in order to "stop" someone with a knife, you're going to have to inflict some major trauma; either damaging large muscles & tendons, severing arteries, or getting into the lungs and/or heart and even then you probably won't drop them instantly. This also raises a relevant question: is your son able physically (training) and mentally (mindset) to kill someone if necessary. It seems that the majority of people that carry weapons or train in martial-arts for self-defense never really "think through" these issues.
An effective addition (or alternative) to a knife might be an impact weapon of some sort (kubaton, koppo, etc.). Impact weapons have a better potential to "stop" (stun, K.O.) someone than edged weapons. I feel that they're also more intuitive and easier to train with. Another nice thing about impact weapons is that there are many objects that can be used as improvised weapons. In Missouri, based on the rather broad definitions in the code, any type of purpose-designed impact weapon is technically illegal. However, a small flashlight like the Mini-MagLight or even a Super-Sharpie Marker make excellent IW's (and they're innocent, I immagine he could even carry them in school).
So that's my $0.02. My advice is that, regardless of what direction you take, get him some training in something. Any increase in awareness and/or skill is a good thing. I hope I didn't come across as being critical, it's great to see a parent who has enough concern for their child's safety to actually do something about it.