Animals should not have to suffer at the hands of poor zoo conditions and care. This polar bear is alone, dirty or stained with feces from what looks to be an injury to his back legs or possibly paralyzed. This is no way for a polar bear to live.
As a species they are known to have access to large spaces where they can hunting, walking and run. They are social animals within their species and require community. The zoo provides the minimal standards for a living environment for this poor animal.
Please make the Belgrade Zoo give up their rights to this animal and release him to a suitable animal sanctuary that can provide him/her with better care.