Sorry to hear about your dog. They have a way of really being a part of the family. I have heard of a product called http://www.dinovite.com
and it claims to really help dogs get the macro nutrition that they need. A couple of other things you might wanna try are from Dr schultze http://www.800herbdoc.com
I would try a Bowel Cleanse mixing some #2 with his food for a week,(I mix it with scrambled eggs) and I would also get some of his "Brain tonic", and just squirt it in his mouth. you will need to adjust for weight, I think the average dose is for 150# so you can adjust accordingly. I have not tried any of these but worth a shot. When it all boils down to it keep one question in mind. Am I doing it for the dog, or am I doing it for ME. Answer truthfully from the heart. If it is the later then it is time for you to repay the many years of companionship the dog has given you and be strong. Pay attention to the dog when the time comes he will tell you enough is enough. It is never easy I have lost many over the years. Good luck.