Hello. Thank you for posting your query at DoctorSpring.com
Dear Dean and Jodie,
I can understand your situation. Firstly, am not really sure if your son has pneumonia. The chest radiograph is grossly normal. Not all fever, cough and rapid breathing are bacterial pneumonia. Such symptoms can also be seen in viral pneumonia (that do not respond to antibacterial drugs).
Having said that, let us assume that your child actually has bacterial pneumonia. The choice of antibiotics would depend upon the organism or infectious agent that we are trying to target. Then, giving him erythromycin or azithromycin would not be wise according to me. These two would largely cover for chlamydia and mycoplasma. Giving amoxicillin-clavulunic acid to him would be the best option. Your child being 12 kg, you can give him 40 mg/kg/day i.e. 480 mg per day in two divided doses for 5 days. There is absolutely no rationale in combining erythromycin and azithromycin. If you wanna give only one of the three listed, then you can use Azithromycin.
Giving cetirizine would not be of much use. Kindly get back to me in case you have any other clarifications.
Regards