I do not have any experience with degenerative disc disease either but here's some info .....
I agree with the advice the other posters have given you. As far as the glucosamine supplements I would get a reputable brand that also includes chondroitin and msm. I have a 95 lb yellow lab that gets 2 capsules twice a day with both his morning and evening meals.
Salmon oil is another good supplement to add. If the brand doesn't include vitamin e in it them you need to supplement that a few times a week as well.
The link TeriM gave you ( http://www.dogaware.com/kidney.html#protein
) has excellent information. I would definitely switch your dog to a grain free kibble and recommend Orijen ( http://www.orijen.ca/orijen/products/
) or Acana Provincial ( http://www.championpetfoods.com/acana/products.php
Good luck and please keep us posted.