I have Scotties, a breed known to have wonky liver enzyme levels. There is a very good liver support supplement called Denamarin that I used in one of my dogs who was suffering from hepatitis. It contains both milk thistle and SAM-e. My dog had two levels in the 1,800 to 2,000 range. Six weeks on the supplement dropped the levels to 1,000 and 1,200. She was very ill from other health problems so I don't have long-term results, sadly.
I have also used Denosyl with success as well. My first Scottie had an Alkaline Phosphatase level in the 700-900 range most of his life. My vet would have him take the supplement and then retest in 6 weeks and again at 6 months, just to monitor.
Links to the products mentioned:
If you are not seeing any signs of liver problems and the dog has a good appetite, I don't think you would be wrong to try supplements and diet change and retest in 6 weeks. Both my family vet and my critically ill dog's internal medicine vet used the 6 week guideline to see if the supplements were working. Good luck. And please keep us updated on his condition.