My four month old puppy will not eat, potty train, or listen. HELP!?

My fiance and I got a golden retriever puppy when he was 8 weeks old. At first everything went fine. We got a crate and are trying to crate train him. He did great for a while, he even got to the point where he would not mess in the house. He had also learned sit on command, as well as could play fetch. Then, about two weeks ago he got wild. He refuses to use the restroom outside. He will not eat his food at all. (The vet says he is really under weight and its making me very worried) He now chews up everything and refuses to listen to no. He will not sit, or play nice instead he tries to chew on you. He terrorizes the cat (which he used to get along with) and he wakes up at least three times every night whining to go out. What do I do? This cannot continue! We are exhausted and our furniture is paying a hefty price. I have had golden retrievers from puppies before and understand that they need to chew and play but never one with such horrible behavior issues. Please help :(