Dogs has hacking cough that sounds like something stuck in his throat, what can I do about it?

8 weeks ago we took in a 12 y/o rott from these people who lost their home. We had a vet come to the house a few weeks ago for a check up & get him up-to-date on shots. About 5 days ago he got a hacking cough, like something stuck in his throat. The vet has him on matacam & tramadol (because of his age & severe hip dysplasia it's now just about keeping him comfortable). Is there anything I can do about the cough? Having the vet come over is not an option, we are already paying for all his meds (inc. Heartgard & Frontline) & food with no help from the owners and the owners cannot pay for a vet visit (they have not had him to the vet in 4 years and we paid for the house call). Other than coughing he's eating & drinking normal and seems content, even his stubby little tail still wags when he sees me.