Hello sivi. I am sorry to hear of your dog's kidney problem. I don't have much experience of this in dogs although one of mine had renal failure towards the end of her life. She was older than your dog and had other problems so I took the view that as it was terminal, she should have what she wanted. However, diet is important in controlling the disease process and in helping the dog to feel better so I understand why you want to get it right.
The problem is that kidney disease can cause diminished appetite and this may be why your dog is not interested in the special renal diet. BTW which one have you tried? Is it the Royal Canin Renal dog food
Regarding his lack of interest in the commercial food, have you tried pouring a little bit of warm water on it? Sometimes the aroma encourages them to eat and warm water can help with this. Alternatively, I am wondering if you might be better off making your own. You need to be looking at restricting protein but what you do give should be of good quality. There should be no added salt. The key features of a suitable renal diet are outlined in this document
which is produced by Royal Canin. In making your own food for your dog you cannot hope to achieve the exact quantities and nutrition that is given in commercial food but it may be more palatable. Obviously you would need to discuss it with your vet but the type of diet may include things such as chicken and white fish for the protein, brown rice or oatmeal (both well cooked in water) and suitable vegetables. Omega oils can be given as a supplement - again, check this with your vet.
If you check the ingredient list of the RC renal you will see that it is bulked up with a lot of carbohydrate in the form of rice, maize and beet pulp. You could do a similar thing but with different types. Manufacturers use rice, oatmeal, potato and sweet potato as carb sources. Of course some of these also contain protein so it would be best to check with the vet which of these would be best, especially as it would form the bulk of the diet because of the lower protein. You may need to do your own research regarding suitable ingredients, bearing in mind what you want to achieve. In the aforementioned document they speak of restricting phosphorous but there is one thing that they don't mention but which needs to be restricted in human renal failure and that is potassium. TBH I do not know whether this is an issue in canines.
If this seems too complex, the only thing I can suggest is that you ask the vet to put you in touch with a nutrition specialist vet or veterinary nurse who could perhaps advise you over the telephone or Internet.