Cat has cold that won't go away!?

I adopted my cat Boo Boo (5 months) 2 months ago at the pound. Everything was fine until 2 weeks after that. He started sneezing and coughing and getting congested. I took him to the vet and he apparently had a respiratory infection. I gave him all antibiotics as instructed, and he was fine. One week after that, he started coughing again and sneezing. I took him to the vet and they said he had another respiratory infection. I gave him meds once again, exactly as prescribed and he was fine for a week. He seems congested in nose area only now, and has started coughing/sneezing once again! I can't afford to bring him to the vet every 2 weeks because the bill is about $100 each time, plus it is very hard to give him meds 2 times a day alone. Should I go to another vet? Could it just be allergies? Any ideas?? Please help!!!! I just want him to get better! PS. He is eating normal, drinking plenty of water, VERY active and playful. The only thing wrong is the congestion.