It depends if the heart murmur
is innocent or not. Providing it's not affecting the hearts performance/function then I can't see why it would be a problem. He'd need to have further tests starting with an echo.
The distance he can run is irrelevant, although it does indicate that his heart function isn't reduced or abnormal.The more serious heart murmurs
, for example those with significant valve leaks or narrowed areas at the valves, may present as reduced exercise tolerance- which he clearly does not have, so his ability does bode well in his favour.
Heart murmurs 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.
As I said above, It really depends on the cause of the heart murmur; if it's not causing any symptoms or problems and the murmur is not affecting the hearts performance then it is likely an innocent murmur.
An innocent murmur basically means that even though there may be an abnormality within the heart causing the murmur, unless it's effecting the hearts performance, it is quite normal and no action needs to be taken. Most innocent murmurs occur because blood is being pumped faster than normal or usual.
The Army are rigid though, often if they just seen an abnormality (even a normal varient), they reject automatically. If his murmur is innocent then his GP should write a supporting letter.