Find a new vet. Treatment for a respiratory infection shouldn't be more then $150 at MOST.

I have seen people get antibiotic injections for cheaper.

A fecal float might be worth it to see if he has any internal parasites. A fecal float should only cost around $30.

My best suggestion is to find a new vet :/ Vets who have no clue what they are doing often do more harm then good.