Many dogs and cats live quite a long time with good quality of life with renal disease. Careful management with diet, medications like the Epakitin and close monitoring thru bloodwork and urinalysis will give him the best chances.
If your dog is healthy otherwise and not too bad off yet (it sounds like he's doing pretty good if the Epakitin is the only treatment needed so far) he may have several good years left. Your vet should be able to explain what is going on and give you a prognosis for future health in your particular case.
There have been recent studies that show that fish oil added to the diet can be protective to the kidneys. You may ask your vet about adding this to the diet. A big dog like yours would do well by 2000-3000mg (2-3 gelcaps) of fish oil daily.
Here is more info on kidney disease in dogs to help you understand what your dog may be dealing with:
http://www.vin.com/VINDBPub/SearchPB/Proceedings/PR05000/PR00128.htm
Holistic Wellness Care