My 15 year old some has just been told by the doctor his liver isnt functioning properly, dietary advice pleas

my 15 year old son was in hospital last night and the doctor has said his liver isnt functioning properly, however he wasnt very helpful as to why. Can anyone give me any advice on lifestly changes I can put him through, eg. his diet to maybe help this condition. I know this is now a medical problem which will have to be looked into, but in the meantime, any advice would help. Please please serious answers only, as this is a very upsetting time for us all. thank you in advance