I've never personally known a child who has had pneumonia, but I've had it a couple of times myself. I would be very worried if he is getting worse! I usually felt a lot better about halfway through treatment.
You mentioned head congestion, not chest congestion... if that's the case, then the pneumonia might very well be gone and he might have a sinus infection or a head cold. If he's still wheezing and coughing, you should take him back to the doctor ASAP... but if his nose is stuffy and such, you should call the doctor and ask if he needs to be seen again and if there is anything you can do for the symptoms.
I'm sorry that I don't have very much advice to give. When my daughter was young, the doctor told me to bring her in if *anything* was wrong, because even small things could mean something serious. So that's what I'd probably do... call the doctor ASAP, or even take him to the hospital tonight or tomorrow if need be, just to make sure that everything is okay. I appologize if I sound alarming, I don't mean to be!