Hello Sharr76 - welcome to the forum. I am sorry that your Cocker Spaniel is so ill. I hope that the medication helps. We have an existing thread on renal disease in dogs and this can be found here.
There may be other threads that might be helpful and these can be found using the search box at the top of the page.
You mention that your vet has suggested low protein dog food but hasn't said what percentage, dry weight. My guess is that he is talking about something in the region of sub 21%. Using the Dog Food Directory we can search for something with a protein at that level but mostly the results will be dry food. Some will be low fat/diet products. Dry food causes a period of dehydration in the dog which is not what you want for one in renal failure so if you go for this option it is better to soak it before serving.
The prescription renal diets are the ones that are lowest in protein but they are not the best quality. If your vet offers one of these, wet food could be the better choice.
I see that you are cooking for your dog and I wonder if you would be able to take this a step further? You would need to obtain some recipes from a canine nutritionist. We have some links in the home cooking section. There would probably be a charge for this but once you have them you can rotate the meals. This has the advantage that you are feeding something that is highly digestible.
If you prefer a complete commercial food and want me to search for something, please let me know. However, it would help if you could ask your vet the precise dry weight percentage.