Hi and welcome to hCM.
It can be tumour but this isnt the first thing to think of. It can be swollen lymph
node, cyst or thymus
residue. If he has no symtpoms or pain and non smoker there is no reason to be concerned. But i suggest to do frequent check up or CT scan
to rule out something serious especailly if he experiences some symptoms such as cough, pain , fatigue
. WIsh you good health.