hi there there are lot of things to explain in this but ill try to explain you in short so that you can understand. heart has two sides right and the left, right side of the heart recieves used blood that returns from the body parts and right ventricles pump this blood to lungs to supply it with oxygen again and to make it reusable to the tissues now this oxygenated blood is returned from the lungs to left heart which pumps it back to whole body. thats how circulatation is completed. now coming to your sons case, right ventricle (rv) can dilate due to various reasons and we summaries them in two scenarios Pressure and Volume Overload. pressure overload happens when there is some sort of resistance in the normal blood flow as some lung patholohy and volume overload is due to extra amount of blood entering the right side of the circulation from some abnormal pathway like some congenital heart defect or some valve incompetency of right heart. Your son had episodes of chest pain which is not a good sign, he probably needs a bilateral lower limb doppler to rule out any sort of blood clot which could have embolised from there to heart and lungs leading to pain and a dialted RV and murmur. Good Luck