Speaking as a 16 year old girl i thought i might be able to add from my own experience.
From year 10 onwards i remember over half of my year group doing the same thing; more often than not, it was light scratches that they could then 'accidentally' show off to their friends and cause attention and fuss.
This in itself i guess shows that most of them did have problems because of their crave for attention- but after a few months most of them stopped because people started to realise they had no real reason to be doing it and the attention would stop.
Hopefully this is the case for your son, and now you've confronted him about it, he's likely to stop soon (if he is doing it as a phase) because he won't want to turn it into anything bigger if he hasn't got any real problems. However, if you notice it continues, and he becomes quiet and keeps to himself a lot, then it's worth going and speaking to a counselor outside of the school or your GP.
From my experience with this, and friends who had this phase, schools are awful with this type of situation- unless you have a student support officer at the school, which helped me and a few others. Regular teachers and HOY's are never any help!
Hope things get better for you and your son!