When in doubt, best to have him checked out by a pediatrician. The last thing that you want is a possible undiagnosed condition to get worse. While I don't necessarily believe in doctors tedency to prescribe medications for every little thing, I believe they can in most cases(not always) provide a fairly accurate medical diagnosis. You don't necessarily have to comply with their treatment recommendations if you are not comfortable and get a seek the treatment advise of a reputable naturopath or homeopath. But at least you will have peace of mind knowing what the problem may be.
These are some interesting articles that make reference to what could possibly be causing the problem. I am not suggesting that this applies to your infant. Like I said you are best to see a pediatrician IMHO.