If he does have a liver shunt, you need to get him on a hepatic diet because protein could actually be deadly to him. My yorkie has a liver shunt, and he's on Royal Canin Hepatic Diet. He's a finicky eater and he actually loves this food!
You might try that...it has everything he needs, low protein (and the protein it has comes from vegetables), zinc, it's high in carbohydrates. It also has a lot of other things he needs, but I can't think of everything. Good luck with the bile acid tests!!! I hope they come back below 25, like they're supposed to be. Let us know how the results come back, and if you have any other questions, don't hesitate to ask, I'm dealing with the same situation right now and have been dealing with it for the past few months.