My 18 month old son keeps getting a fever as soon as panadol (acetaminophen) wears off its been 3 days he has a dry cough no runny nose dr said there was no infection?

Further workup d5: If your son is still having a fever of 101 degrees F or more for five days, I would suggest a further workup to include a CBC, diff, plts, CRP. Sometimes the findings of pneumonia may be very subtle and a chest radiograph may provide more information. Most likely he has a viral illness which will resolve in the next couple of days.

Need more info...: Fever that persists in a child should be re-evaluated in person by a Pediatrician or at Urgent Care or ED. The source of the fever sounds like it is in the lungs given the cough. It could simply be a virus, but I cannot exclude bacterial causes such as pneumonia without an exam. Fever with the symptoms you describe is certainly evidence of an infection. Viral or bacterial.

Cold: this whole thing could be cold that resolves in 3-5 days. If no vomiting, breathing difficulty or rash, it ok to watch it if he is drinking ok