Small animals in general are not very "strong". They are prone to getting diseases and cancers and will hide them up until the very last moment when the pain becomes too bad to cover up. Prey animals will always try to hide weaknesses to survive a little bit longer.
There's no way to narrow down what type of disease it may be. Virtually anything that could be causing him pain. Kidney disease, liver disease, arthirits, cancer, overgrown teeth, you name it. There's no way to diagnose it over the internet. At the very least, go to the vet and get some handfeeding formula so that he's not starving to death.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry