Hi, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. I'm so sorry to hear about Rastus being so ill!
Many different diseases can cause coughing, lethargy, and pain. Some are emergency situations like heart failure, diabetes, urinary obstruction, etc. Is Rastus able to urinate normally? For how long has he been ill? Has he been drinking more water than usual for awhile? . . .and. . .What color are his gums?
Most vets will provide emergency care. . .either by their own clinic or through referral to an emergency clinic.
I'd like to try to determine if he is stable enough to wait for veterinary care, and to determine what you can do for him in the meantime.