My son has hypoplastic left heart syndrome. His liver is enlarged and his abdomen is distended. Is this due to diastolic dysfunction?

Perhaps. There are multiple causes of liver enlargement and abdominal distention following fontan (i am assuming that he has had a fontan procedure.) my may have heart failure, diastolic dysfunction, obstruction in the pulmonary arteries, obstruction in the fontan circuit, or tricuspid insufficiency. Also, these findings may be signs of protein losing enteropathy. He needs a careful evaluation.
Yes in part. Hypoplastic left heart syndrome occurs when the left sided chambers are radically underdeveloped resulting in inability to adequately deliver oxygenated blood from the left heart to the body. This can result in back up into the lungs/right-sided chambers . A distended abdomen/liver engorgement suggest right sided chf. This can occur in a hypooastic left heart.