My dog is also on cartrophen for life. I chose not to have the surgery as he did not completely tear the ligament, but it did mean a long time off, and now he is restricted again (he is older though).
I just feed him Acana Pacifica and also a glucosamine/chondroitin supplement.
For weight loss, I would start with a little bit less than the recommended feeding and go from there-you may have to decrease or increase. Adding pure pumpkin puree is a great way to add bulk (if he is not satisfied with the amount of food he is getting) without adding calories (plus it has many other health benefits). I add it to both of my dogs' meals (and it's great for putting in kongs, and for getting them to take pills. Great for their digestive systems!