They're hard to see. If you are making a self-diagnosis, stop and let your doctor take over. We understand that it is a major illness. Often viruses are the cause of myocarditis. Most patients make a complete recovery from myocarditis.
Rest is their treatment.
For a few, the heart muscle inflammation is so great that their hearts fail. If that does occur, the heart failure is treated, and many then go on to complete healing. In fewer still, heart damage occurs but does not produce any symptoms until later in life. These myocarditis patients are liable to develop a kind of heart failure somewhat difficult to treat. Your grandson should be able to achieve complete recovery.