Your child might cut or hurt himself and really just be fooling around; but it is unlikely. Even then, it will happen once or twice and stop. If the cut or injury is more than minor or if the behavior persists, your child has a problem.
Talking with him about the behavior and listening carefully for other signs of maladjustment helps. This is a very serious problem; and if it happens more than once or twice, the young person needs a mental health evaluation. Also, look very closely for other signs you may have overlooked. With emphasis, be sure you do not accept the explanation you may receive from others that he is doing it for attention. It is much more serious than that.
Although self-mutilating behavior is most likely related to mental illness, it may have other origins. For example, the behavior may be related to gang initiation activities or may be a fad in which youngsters in an area are participating. Whatever the etiology, pursue the behavior until you understand the "why" of it and until it stops.