If he's not getting better and it's been 6 weeks, he needs different antibiotics. I would take him back to the vet, get a culture done and see what it says is the best antibiotic to put him on. Most antibiotics are every other day, not every 3 days(too long of an interval for them to work).

He doesn't look very good.. His back end is distended and he looks really dehydrated (the lateral folds in his skin). I would start soaking him every day for 30 minutes to an hour. Get a 6-15qt plastic container with a lid, put some paper towels and about 1/4-1/2" water, put him in, close and secure the lid, and put it on the warm side of his enclosure.

What are his temperatures and humidity measuring? They need to be bumped up to around 95/85 with about 65-70% humidity. Higher temps and higher humidity will help loosen the stuff in his lungs and let him breathe better. Higher temps allow his immune system to work better. Make sure his temps stay STABLE 24/7, no drops at night.

Like I said, definitely get him back to the vet for a culture. The antibiotics obviously aren't doing their job.