Is your father competent to understand the issues and make health care decisions for himself? If not, did he previously make his wishes know to you? It is a very personal decision to treat vs not treat. The most common form of kidney cancer is resistant to conventional radiation and conventional chemotherapy, but responds to the newer "targeted" drugs, at least for a time.
The fact that he is not a candidate for surgery will tend to shorten the time he has left. Nexavar is relatively new, and the data accumulated so far indicates that Nexavar will likely extend his life in terms of months, not years. Since he is comfortable now, you might consider having him try Nexavar to see how he tolerates it. The severity of the side effects may help you to decide whether to continue the Nexavar; some patients tolerate it well, but some do not. Also, these new drugs can be quite expensive, if that is a consideration.
You sound like you are a thoughtful person. I am sure that you will do what is best for your father.