BPJD36, I am sorry your dad is so sick. If his doctor is not trying to find the cause of his symptoms, please get him to another one.
Has he ever been bitten by a tick that you know of? If he has been tested for other things with no results you may want to consider Lyme disease. It is a multi-system bacterial infection caused from the bite of an infected tick. Ticks can transmit several different diseases. No test is completely reliable, so it is very important to see a Lyme knowledgeable doctor. Lyme disease is the second fastest growing infectious disease in the US after AIDS, yet many doctors are unaware of its prevalance or seriousness.
If you want to post where you live, I will give you the name of the closest knowledgeable doctor to you that I know of. For more information on tickborne illnesses, see the "Lyme Disease Information" post on the Lyme board at this site.
Keep searching, you will find answers. Your father is very lucky to have you helping him