First let me say I'm so sorry you are going through all this
I read your post and didn't read everyone's replies but
I had to mention:
Why would the vet prescribe A/D canned food for your puppy if he is showing possible signs of Liver Shunt? I have given A/D to my healthy dog when he was tired of eating the same food. I have a yorkie with mild MVD which is a type of shunt that can be treated with diet and meds. He's doing great. A/D is very high in fat/protein and it's the last thing your puppy needs if he is showing these symptoms.
When I first got Cookie ,he was very picky and tiny.He didnt gain weight well.I did not know anything about Liver Shunt at the time. I tried giving him so many types of dogfood and he did not want any of them. The breeder then said to give him chicken and rice.That worked for about 4days.Then Igave just chicken alone and he refused that as well.My vet at the time never mentioned Liver shunt. I even gave him A/D and he gobbled it up of course but was much more lethargic after the 2nd day
Thank God I changed vets when he was a yr old and we found out it was MVD and started him on a Science Diet (very low protein) Actually he can have up to 20% protein and be symptom free. Of course this is a Mild case.By then it was too late for the Puppy Lemon law . I'm sure you are already in love with him and returning him may be something you will not consider.
There are tons of sites including this one that share lots of info on Liver Shunts. It can be scary once you read of all the possibilities . But if you are well informed you know your options. Talk to your vet again and see if he agrees that your furbaby may need a Bile Acid test to begin with. There several types of shunts ,some can be operated ,some cant and some can be treated simply with a diet change.
How much does your little guy weigh? Mine was about 1. 5 lbs when I got him and he went down to 1 lb. Now he's 4 yrs old and weighs 3 lbs even
Good Luck and keep us posted on your baby.
Genie
Cookie,Lola, Angel & Lucky