Thanks for the extra information.
I could probably sit here for quite some time and list all the disease and processes that can cause these values to increase and am happy to do it for you but I will start with hitting the most likely scenarios in your dog:
1. Liver Disease: Dogs can develop diffuse diseases of the liver that include hepatitis, stasis of the gall bladder, fibrosis, cirrhosis, etc.
2. Liver Cancer; This unfortunately has to be on the list. We can often see just mild, non-specific elevations of these enzymes when something is growing off the liver. Now, these growths can be benign or serious and sometimes even completely cured with surgery.
3. Gall Bladder Disease: The gall bladder can get infected or inflamed as well. Also, there is a condition called a mucocele where bile hardens and can cause the gall bladder to distend.
4. Normal liver regeneration: Unfortunately, these values can also be "normal" in dogs as they get older due to chronic regeneration of the liver itself. I have seen dogs with ALP's in the 1200-1500 range that we cannot find a thing wrong with them.
5. Cushing's disease: Probably not likely in your dog due to the age but always a possibility.
I would start with an ultrasound of his liver and move from there. Often, non-specific therapies to remove bile and cleanse the liver can be effective once we know what is going on.
I hope that helps! Please let me know if you need anything else!