Thanks for posting your query here.
It is difficult to pinpoint exact cause for this issue. There could have been an injury
or sometimes a virus/bacteria infection can also lead to infection of the joint leading to swelling and restriction of the movements.
Is he been running any fever?
Has he suffered any recent infection like cough/cold/diarrhoea-vomiting?
I would suggest continuing same medications for now. If things still not settle within next 48 hours, your son needs to have a Ultrasound
scan of the knee and hip joint to rule out inflammation/infection of the joint.
Please do revert back if things don't improve or if you have further queries.
Dr Tejas Shah