M.G.
Not all pneumonia is caused by a bacterial infection (what you use antibiotics for). Some are viral. If he's getting sick again after being around people with colds (virus) ... it's possible he's got a viral pneumonia and just being exposed to viruses is setting him off again.
What a sin. He must feel awful ... is he really sick? When we've had pneumonia here we were in bed.
Surprised they haven't sent you to a specialist by this point. Hopefully your pediatrician will make some calls for you and get you in soon.
There are other options to draining the fluid etc. but would involve a specialist.
My kids have asthma after colds - but of course we don't treat that with antibiotics. That has lingered for a while but it's more of a wheezy cough like condition, not fluid in lungs.
I suspect once you get in with specialist, they will be able to pinpoint what is causing it and get you on proper treatment right away. As for 4 antibiotics, yes .. unfortunately that can cause infections elsewhere (I had a kidney infection from being on antibiotics once).
Hope your little guy gets sorted out soon :) best of luck