Crystal - WOW you got it all perfect in one shot
I agree with all posted above. A proper diet change, along with the Metro and Lactulose is ONLY going to make his liver's job less stressful. IF he has a Liver Shunt or MVD, his BAT will always be high. It may be within a range, but it will never come down to normal.
I don't think you should let this go. I think it is great that the new food is making him feel better, but it doesn't change the fact that he may have shunt or MVD. Also, as Crystal said above if you are feeding the L\D dry, you should be adding water (I think preferably distilled water) to the kibble. Or, you can get the L/D wet for more added moisture.
The Protein C test can be done by your Vet, but they need to contact Cornell Unv for the tubes and instruction on the draw, then they have to be overnighted to Cornell for the testing and results.