Thank you for your question. My name is***** I have been a small animal veterinarian for 14 years and I'd like to help with your question.
Elevated liver enzymes in older dogs are a pretty common finding. They can be caused by a number of things... sometimes simple aging changes in the liver will cause it and it is nothing to be worried about. It can also be realted to medications if he is on any medications (anti inflammatories are the most common to cause this). It can be related to hormone abnormalities, such as cushings). Cushing is probably the most common thing I see. Cushings causes a few other symptoms that I usually ask people about... such as increased appetite, thirst, and decreased muscle strength. Are you seeing these symptoms?
The problem is that elevated liver enzymes can also be see with very serious conditions, including cancer. You would have to do more tests to know if this is something serious or not. I usually stop any medications that they were on prior to the enzyme elevation (it doesn't sound like he was on any), test for Cushings if there are any symptoms of cushings, and consider a liver ultrasound to make sure there isn't a tumor. There is also a test called a bile acids test that can test the actual function of the liver that I do sometimes. If all this is normal, I treat with denamarin and metronidazole like your veterinarian is doing.
So, I think you are on the right tract for now. The only thing I would consider is the cushings test. If there are no signs of heart disease (no cough) then I would hold off on heart medications. However, if he is coughing I think the heart medications are indicated even if the liver enzymes are not sorted out.
I hope that helps, but if you have more questions, let me know. If not, please don't for get to rate my answer.