I am an ex self harmer and what your son is doing doesn't sound like a prelude to anything, just frustration, same as a toddler might bang his head. Most people at some point in there life will have done something similar out of sheer frustration. I do think though that if given enough attention for it it has potential to turn into more just as any problem can escalate given the right/wrong attention. Having said that though, if you have believe he has reason to turn to self harm you should take him to a doctor and get advise and help him learn to cope with his frustrations better as unfortunately life is full of them.