hi findthetime,
i remember when i was young about 12-13 yrs old, my best friend would self harm with metal rulers, compasses, knifes. she used to do when she would get a bit stressed out (and yes there are stresses for 12yr olds!!!).
she didnt always hide it, she would do it in front of me and do it on her arms/legs/hands. she used to act like it was perfectly natural even though it used to confuse me.
looking back i can see it was her reaction to pressure, whether it be school work or when she found social situations difficult, simple things like making new friends or talking to boys, could bring on a bout of self harm. fortunately she grew out of it before it got serious.
my advice would be to keep track of him, try and talk to him and make him feel relaxed so he won't get defensive. definately don't grill him! encourage him to spend time doing social activities he enjoys. if your son is closer to another relative, brief them on the situation and get them to talk to your son.
good luck, let us know how things go.
x