
My 15 month old DD does the same thing. It's awful, because you take such great measures to make sure they don't get hurt and then they bite themselves, gouge their own eyes, pull their hair, scratch themselves, etc. DD does it when she is stressed--like when she's overtired or overstimulated. Or really fighting sleep hard. Or mad at me because I won't let her bite my arms.

I'm not sure why they do it. I'm getting DD an OT eval for the combination of these and some other sensory-seeking behaviors, but if this is not a sample of some bigger trend, then I'd say he's just expressing his frustration. Kind of like an adult punching the wall, I guess.
I'm anxious to see what others have to say....