Is there anything wrong with this ferret?

It just sounds like some flies have laid their eggs in the mess, while he has been gone.

Kat is right but ferrets are not considered "caged" animals. They are not like hamsters or gerbils. They are more like dogs that require a lot more stimulation a day than fish tank mice. Please pay attention to this one or the ferret will be self destructing in its alone time. It is recommended they have 4 hours a day of play time outside of their cages. These worms in the poop are not from flies laying eggs. The ferret has worms inside its digestive system. I have seen this in cats and after first seeing it in the stool samples in the litter box, I eventually saw them crawling right out of their butts. Take these pets to the vet for some de worming meds.

He most likely has worms the mites could have been living inside of him but living in a cage of his own filth for that long could increase the chance of them crawling back into his system. You should really have cleaned that animals cage he could be suffering from his lack of care and you should definitely become more aware of animal and cage hygiene before getting anymore caged animals.