are due to abnormalities and disturbance of blood flow within the heart and vessels. Most are benign, normal variations in the structure of the heart and cause no problems. Sometimes a murmur can indicate a problem with the heart valves, it can also be caused by holes between the pumping chambers, or just by turbulence in the blood flow- which is generally not a problem.
The Doctor will do an Echo + other test and providing it's not causing any symptoms or problems and the murmur is not affecting the hearts performance then it is likely an innocent murmur and no further action needs to be taken.
As your son is experiencing symptoms it will need to be assessed whether or not it is cardiac related. If it is to do with his murmur then he will be given treatment accordingly. You'll have to wait and see what the tests reveal- try not to worry too much. Heart murmurs are very common.