First of all he should get further testing to figure out the etiology of his abnormal liver tests. He should get tested for viral hepatitis
, autoimmune liver disease, etc by blood tests. He should also get a liver ultrasound
to check if he has fatty liver
or stones in the bile ducts.
If he drinks alcohol he should stop completely. There is no magic diet to decrease the liver enzymes. If he has fatty liver disease then he should eat a regular and balanced diet, low in fat.
He should definitely get follow up with a liver specialist or gastroenterologist.