If you want to know what is truly best, I would highly suggest switching him to a raw diet. I have heard a lot of stories about dogs who are so far gone with lots of different problems (cancer included) that make a 180 degree turn around. If you are really interested I can direct you to a wonderful Yahoo! group that has thousands of members who have been feeding raw from anywhere to 1 day-20 years and have seen awesome results. They have been so helpful for me with all my little questions about what I can and cannot feed, if I'm feeding the right ratios, etc.