Thanks for writing your query.
Pain in legs is common after high grade fever
. This is known as myalgia
It is due to some biochemical changes in the muscles as a result of high grade fever.
Pain will subside in due course of time(few days to weeks).
Give him proper rest. This will speed up the recovery.
He should be given nourishing diet to compensate loss of electrolytes due to fever.
Hot fomentation and local application of pain relieving gel will be helpful.
You can continue giving him pain killer for some relief from pain.
If the problem persists, get him examined by his Paeditrician.
I hope this is helpful to you.