I am so sorry that you are having this difficult experience. Unfortunately, I do not think that I can provide good news.

There are a number of viral illnesses that can cause high fever and subsequent neurological damage in ferrets. These viruses are unfortunately common in breeder's facilities and pet stores. Some viruses replicate and express themselves quickly in a sudden fever, but some can incubate and cause damage over a great deal of time. Other illnesses that cause fevers in adult ferrets (intestinal blockage, for instance) have additional symptoms.

Here are some links to things to discuss with your vet:

http://www.miamiferret.org/fhc/aleutian.htm This illness can take up to 2 years to fully express itself...fatalities are to be expected in the majority of cases. Paralysis starts from the back-end-forward.

http://www.veterinarypartner.com/Content.plx?P=A&A=674&S=1&SourceID=43

http://www.ferretnews.org/influenza.html

http://www.cvm.uiuc.edu/ope/enotes/showarticle.cfm?id=181

I am going to also include these links to sites that discuss ferret illnesses and have vet-search indexes...you can either have a second opinion of the tests already run, or perhaps find a ferret-familiar vet who is open to giving your regular vet some advice:

http://www.quincyweb.net/quincy/vet.html

http://www.ferret-universe.com/vets/vetlist.asp

http://www.ferretsanonymous.com/vetlist.html

http://www.ferretnews.org/clinic.html

http://www.ferretclub.com/vets/

http://www.rabbitrodentferret.org/rabbitrodentferret.org/vets.asp?animal=3

In the meantime, do continue nursing this little one, and be sure that he can urinate and pass stool. If his appetite is poor, you can offer jars of baby food meats and fruits to supplement his regular food. Provide a thick plastic bottle filled with warm water (or similar, hard-shell heat source) in case he has trouble keeping his body temperature regulated. A heating pad should only be used under one corner of his bedding (under the cage itself), and set to LOW only...he should be able to easily roll away from heat if he needs to cool off.

As is discussed in these links about Aleutian Disease, this parvovirus can take up to two years to fully express itself. It is most often carried home from the breeder's facility or pet store:

http://exoticpets.about.com/cs/ferrets/a/aleutiandisease.htm

http://www.peteducation.com/article.cfm?cls=11&cat=1283&articleid=1700

Its more devastating effects are more often seen in young ferrets, but adults exposed during an infective stage of the virus life cycle can become infected and carry the virus "silently". This is a poorly understood virus as yet, but goes to the head of the list in a young patient who does not show any other metabolic or cardiovascular symptoms.

Testing information is provided here:

http://www.trifl.org/medical.shtml