Rocking is a common behavior in children, Kristen. It's known as a self-stimulating, repetitive behavior. Other similar behaviors include thumb sucking, hair twirling, finger wiggling, masturbation, arm flapping, head rolling and head banging. Young children most often do these behaviors when they're upset, bored, concentrating on a task, listening to music, watching television or falling asleep because it tends to calm them or help focus their attention.
Most children who rock tend to outgrow it by preschool or school age. Although rocking is usually normal, it can sometimes be associated with other developmental and behavioral problems such as autistic spectrum disorders, developmental delay and attention deficit hyperactivity disorder. It sounds like his development is on track, but it's good to discuss your concern with his doctor and have him do a complete developmental screening to assess all of your son's skills and behaviors. If you're concerned about the possibility of autism, you can ask your doctor to do a specific screening for autism. This would check for other possible behaviors of concern such as delayed social skills (not making eye contact, not looking at objects you point to and not making friends), language differences (losing language skills, not wanting to communicate, speaking in monotone or echoing), and unusual behaviors (injuring self, playing with parts of toys rather than the whole toy, not doing pretend play and having unusual intense interests).
If your doctor determines that your son's rocking is normal and not associated with a developmental or behavioral problem, be sure to discuss how you should respond to the rocking. Most pediatricians recommend ignoring the behavior and waiting for it to disappear on its own over time. Criticizing or punishing your child for this behavior may increase his stress and lead to the need for more rocking. If you feel that his rocking is getting in the way of his interacting with you or others, you can try to gently get his attenton and engage him in positive interactions such as playing together or going for a walk.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.