Older cat with problems.

Hi there. I really hope that you can answer my question. My cat Derek is obviously sick. For the past month, he's been sick. We first noticed something was wrong with him when he went to jump on the couch and he could not! He fell over. Then we saw him walk, and he was walking like he was all disoriented and dizzy. He stopped eating. He lost a ton of weight and was drinking a lot of water. His little head would be hunched over in his water bowl. So, we took him to the vet and we could not afford almost 400 dollars to have all the tests done. my husband and I are both unemployed at the moment. The vet gave us some amoxicillan and some injections. We gave him those for a week. After a week, he seemed like he was getting better. He was jumping on the counters, walking normal, eating again, but eating ALOT. He would want to eat ALL the time. He would follow us around wanting food all the time, so we thought that he was getting better. Well, for about 2 days now, he took a drastic change again. =( He will not eat. He is walking like he is drunk, and it seems as if his back legs are weak. He is pacing back and forth, like he doesn't know what to do with himself. He seems like he gets tired from walking, and rests in the middle of the floor all the time. He pees a LOT. A MASSIVE amount. He has been vomiting about three times a day. Last night he started to breathe heavily. I could hear him from across the room. He gags sometimes. He hunches over his waterbowl, and just hangs his head over the bowl. We called the vet and they gave us more amoxicillan, injections and an iv fluid bag. We gave him the iv fluids yesterday. The vet said it would flush out all the toxins if he has some sort of infection. Nothing is working so far. =( I have googled like a mad woman over and over again to try to diagnose my cat, and I have come up with Chronic Renal Failure, but Im not sure. I dont want to put him down if he isnt in pain, but he just doesnt act healthy or seem happy. Please give me your thoughts. Please tell me what you think this might be. It feels as if we are just waiting for him to die. =( I appreciate any help that you can give me. Thank you for your time, and thank you or what you do.