My son is 7 months and I just found out he has an enlarged heart! What can happen? Is it harmful or fatal?

Time for consult. The best source for this information will be the team that sorts out the cause of the enlargement. There are several possibilities. I would expect an echocardiogram, ECG and chest x-ray. These will be assessed & a diagnosis made. Only when you know the cause can you approach treatment & understand potential outcomes.
It depends. There are many cause for an enlarged heart, some of which are more serious than others. Some can be treated with medication and others that require surgery. She should undergo a complete evaluation by a pediatric cardiologist who can then more precisely define her condition, the treatment and the long term prognosis.