Abhir's Health

Can someone explain me, what exactly is the issue with Abhir's health? Is it some health issues within the heart? And Abhimnayu is the only doctor they know? Or is it something like they need matching blood/something which can only be given by Abhimanyu as he is related to him

Abhir has a hole in his heart. It is most likely a conventional heart disease which went undetected at birth. And now it has started hampering with Abhir’s day to day activities, resulting in shortness of breath and general fatigue because his heart is not functioning at it's 100 percent.

 Well, it can be possible that they show Abhimanyu is the only cardiologist which specialises in such cases ( Pediatric Heart Defects) and has the highest success rate. 

They don’t need matching blood or any other donor parts. They just need to patch up the hole which is generally done through cartilage taken from elsewhere within the body (as per my knowledge - I am not sure till what extent it is medically correct)