//////he has a shadow on his heart.///
If we take that statement as gospel, then that could only mean a pericarditis ( fluid around the heart) which could be a result of Covid......but unlikely.
A more likely scenario is that of a shadow NEAR the heart and that is the most likely state of affairs, in which case we are thinking of a Aneurism or enlarged lymph gland which would bring lung cancer into the picture.
From what you have described it makes a clot unlikely.
He will need further investigation of MRIscan of his lungs and almost certainly camera insertion (bronchoscopy )