Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your pup's liver is failing and I can understand how upsetting and puzzling this is in such a young dog.
Acute liver failure, in order of likelihood at this young age, can be due to:
1) exposure to toxins, drugs or chemicals (xylitol which is an artificial sweetener, or acetaminophen, or eating onions could be possibilities),
2) a viral infection (adenovirus is possible)
3) or a bacterial infection (leptospirosis)
4) less commonly fungal or protozoal infections
5) congenital (born with an anatomic malformation) liver disease, such as a portosystemic shunt
6) heat stroke
7) inflammatory diseases where the body attacks itself (less likely at this age)
8) infiltrative diseases, where normal liver tissue is replaced by abnormal tissue or substances such as copper (copper storage disease) or cancer cells. Less common at a young age as well.
A liver biopsy to determine the cause of your pup's disease process would be very helpful, but a transfusion to replace clotting factors and blood proteins is essential before taking a biopsy in such a severely affected pup.
If we support the liver and treat the primary cause in some cases the liver can regenerate if we catch things early enough.
Best of luck to your pup, please let me know if you have any further questions.