Hi My 5 Year Old Son Has Had A High

Hi my 5 year old son has had a high fever for the past 5 days, complains of headaches and sore throat. We have taken him to the Dr and he said it is a viral illness or the flu. Lots of bed rest, fluids and pamol/paracetamol. Last night and this morning he has complained of sore legs with pain behind his knees and not being able to walk. He's grumpy and talking to us but the leg pain is weird. Is this something that needs urgent medical attention?