After hearing the description and the age, my guess would be that the ferret may have insulinoma, which is cancerous nodes on the pancreas. Ferrets with that disease often act that way. Very lethargic, staring into space, hind leg problems, losing weight. They usually eat, drink, poop, and pee normal, just act weird. Do some online research on this to see if that is the cause. Insulinoma can be treated with surgery, but often times it will come back after 6 months to 2 years after the surgery. Prednisone is a medication that also can be given as well. It won't cure the problem, but it will help the ferret to get back to normal.
It could also be other medical problems as well. Ferrets are prone to many issues. Cysts on the liver or kidneys can cause these symptoms as well. Sometimes they can be removed, and sometimes there's nothing you can do. We had a shelter ferret showing all the symptoms you described and he ended up having a liver cyst that was inoperable.
Feel under the back legs, front legs and on the neck. If the lymphnodes are enlarged, the ferret also could have lymphoma. Most of the times ferrets will put on weight with lymphoma, but occasionally ferrets will lose a lot of weight as well. Towards the end of that cancer, ferrets will act the way you describe.
Basically the best thing you can do is take the ferret to the vet ASAP. And make sure the vet is ferret knowledgeable, otherwise you might not find the real answers. To find a really ferret knowledgeable vet join the FHL, which is the Ferret Health List. It is a yahoo group dedicated to ferret health. You can ask questions about your ferret's condition, and you can also ask for info about good ferret vets in your area. There are many vets, shelters, and many experienced ferret owners that can help you out.
I hope your little one gets better.