Dog can't breathe, can't stand up and falls on his side?

Let me tell you the whole story. A few months back, my dog (13 years old, Smooth Fox Terrier) started coughing. He was completely able to breathe, there were no problems like that, but every 10 minutes or so he'd cough, ending with a hack/retch. We, at the time, were unable to take him to the vet. Then one night, he was laying on his side when he started howling, as he would if we left him alone (as if he were anxious, or something), so I ran over to check on him and he was laying on his side. He was in no pain whatsoever. I propped him up, he coughed once, and then went on his way. Ever since, he has been unable to get up when he lays down on his side. Soon afterwards, he started having some breathing problems. He sounded raspy/crackly when he breathed. When that started happening, we took him to the vet with the help of a family member. He got 3 meds that we started having to give him daily. Baytril tabs, 68mg once a day; Prednisone 20mg, one half tablet for 7 days, one quarter tablet for 7 days and then one quarter tablet every OTHER day for 7 days; and Theophylline 100mg, one tablet twice daily. We continued this for a week and had our scheduled check-up. At this point, he was not improving. His breathing hadn't improved, if not worsened. She said she wanted to give him one more prescription, Furosemide 50mg, one half tablet every 12 hours. He's been taking that for a few days now (probably 3 or 4, I don't remember), and since then- I'm not sure if this is because of the medication- his condition has RAPIDLY gotten worse. He's now unable to stand up and walk, his back legs constantly give out, his eyes are open wide, looking alert but zoned out, sometimes eats and sometimes drinks. He now constantly breathes very heavily, as if he can't catch his breath at all. We have to pick him up to take him outside to pee, and when we bring him back in, if he tries to stand up, he just falls over onto his side again. The cough is long gone, but the breathing problems persist and his condition just seems to be worsening greatly. He has lost weight since this began. I can feel his spine now, but I can't see his ribs.