Thank you for your query.
Whenever the body temperature rises very fast one can experience shivering till the temperature stabilizes. You did the right thing by giving your son Calpol
and keeping him warm. You should monitor his temperature and repeat Calpol four hourly. If the temperature persists then you need to consult your doctor to find the cause of the fever
and the appropriate treatment to be started.
I hope I have answered your query to your satisfaction.