There are many things that could be causing this pain, and I am interested in knowing more about the situation. Is your son ill with other problems, or is he otherwise well? Has he had any surgeries in the past, or any recent illnesses such as pneumonia
? Your doctor
, or perhaps a specialist, will want to know more details, so I suggestion booking an appointment right away.
There are several things that can cause rib pain, but the fact that it seems related to movement certainly makes it seem more likely that this is a problem related to his musculoskeletal system. That is, if I am interpreting your statement correctly in thinking that the pain is related to moving his arm. If the pain comes when he himself moves up and down, then that would be perhaps more concerning for something within the rib cage or the lung itself. Also, the quality of the pain can give some ideas as to where the pain is coming from. A very sharp pain with movement of the arm will usually be muscular, while a pain that is more burning but can also be sharp and comes with breathing in or coughing is sometimes related to the lung.
Either way, two months is too long for this pain to be persisting. Please continue to speak with your doctor, and consider seeing a specialist.