It's a Ferret Life

I need help with one of my ferrets. I have two boys and one girl all which were rescued from people who couldnt care for them anymore the latest one a male i rescued he was skinny i could see his bones in his chest he still at and drank water and was very playfull the old owner said she thought it was due to depression becuase his other had passed away but now he isnt eating at all or drinking water and his poop is black and tarry i think it may be stomach ulcers but im not sure. i called the vet a couple times and they cant get me in yet. I have to force feed him and force him to drink. What should i do i think he has givin up on living. Its so sad someone please help!!!!! i made him moshie which is there food mixed with water and he eats it but stops after about 6ml sometime a can get 9Ml and i also tried chicken baby food which he like but thats not enough. any ideas or things i can do untill i get him to the vet he recently started having brown poop again.