I really hate to be too judgemental, but I think a great deal depends on where you live, who his friends are and what he does in his spare time. Whilst I'm sure there may be reasons for someone in inner city London who hangs out in shopping centres/cinemas in his spare time to have a penknife, I can't think of many. If your son does cubs/scouts, or enjoys camping, etc. then there may be more valid reasons.
Having potentially dangerous items can teach children that they are not toys and should be treated with respect as they can cause injury.
DD's school starts bushcraft lessons when they are 7, and the children use pen knives from about that age.
I certainly wouldn't let a child carry it around routinely though.
Incidentally, it's illegal to carry a can of hairspray, if it is your intention to use it as a weapon. Having said that, the carrying of a knife is one crime where there is a presumption of guilt rather than the usual presumption of innocence, and it is up to the carrier to demonstrate that they have legitimate reasons to carry a knife, rather than for the prosecution to demonstrate guilt. At least that is how I have always understood it, quite a few members of my religion use knives as ritual tools, so there was quite a lot of publicity when the law came into being, about how and when to carry knives in public places.