It looks like it's a behavioral thing, meaning he did that on purpose to make him feel good or relax. Most behavioral "problems" go away on their own in several months. If you have great concerns, consult your ped. You may post your question on other sites like Huaren.us or wenxuecity, or some other parenting board (try babycenter.com for instance). Gather information from as many parents as possible, but remember don't do the diagnosis on your own because new mothers tend to scare themselves out of sheer speculation :)