Hello. Those are two very complicated illnesses (I actually have diabetes type 1 myself). There might be a question of which came first - as both illnesses can cause the other. Firstly, kidney disease is not curable, and you need to let your dad know it. The dog will ultimately die from it - or from the diabetes - although diabetes can be of course be managed with good insulin therapy. The difficult thing here is ... kidney disease is largely managed by diet, but diabetes is influenced by diet - so treating one will affect the other.
I feel you are probably right. Any dog with these two illnesses is unlikely to last very long. I wish I could say something more positive. If I were your dad, I would take things day by day, make sure the dog had everything it could want and while he's contented and happy, fine. But when bad days outweigh the good ones, then it may be time for difficult decisions to be made.
Dogs that are in severe renal failure (and to some extent those with diabetes too) feel exhausted, lethargic, nauseous, depressed and sleepy. This is no way for a dog to live its life ... and things are likely to get worse very quickly.
You could start him on renal food (Hills kd canned for example), if he will eat it. Mirtazipine is also a good drug that will help appetite. Anti-nausea medication would help (ask the vet for this). A phosphate binder is also a very good idea. Fluid therapy will also be beneficial (another question for the vet). The trouble is that while this will all help the kidney disease, it may adversely affect the diabetes.