S.G.
Hi, I was a head banger when little haha. Here's an article that states: Up to 20 percent of babies and toddlers bang their head on purpose, although boys are three times more likely to do it than girls. http://www.babycenter.com/0_head-banging_11554.bc
the most heartening part of the article: Try not to worry.
Your toddler may get a bruise or two, but don't worry — head banging is usually a "self-regulating" behavior. This means your child is unlikely to hit his head hard enough to seriously injure himself. He knows his threshold for pain and will pull back on the throttle a bit if the banging hurts.
Good luck