It seems your son has developed Cellulitis
It is tender swelling , warm to touch , the area is inflammed.
This usually results from infection.
It needs a proper management , to avoid serious complications.
I personally prefer to use Cloxacillin
antibiotics. Also , i add a drug for anti inflammatory action like Zynase-D , these combinations have Diclofenac
I would advice you to discuss my opinion with your doctor , i am sure he / she will agree.
Other option is that you may send us the photograph of that area , we would help you for definitive diagnosis.
Hope he gets well soon.