First of all how old is your son?
Secondly to the question you asked, most doctors don't prescribe antibiotics to children unless they suspect it to be bacterial. Infection because most children suffer from viral infection for which antibiotics and anti-allergic medications are enough otherwise repeated antibiotic use without any need will. lead to resistance for future purpose
For immunity purpose I think a good balanced diet is enough for immunity and u can also take multivitamin syrup depending upon the age
Next Steps
Observe symptoms and see whether antibiotics are required or not and keep.patience to check for falling trend of fever for 2-3days along with activity of the child. If the child is active in between the fever episodes and eating well, then you can wait.
Health Tips
If you find your child to be lethargic and less active along with eating less than you might consider of physical examination of the child along with the prescription given by the doctor