It is better not to panic by reading on the net. It these symptoms were following a bout of fever
then probably your boy is suffering from acute costochondritis
....which can be ascertained by pain on pressing on that site.It it is so...give him a paracetamol
tab along with some local anangesis jelly to apply on that site thrice daily...It the pain aggravates on deep breathing..then get a Xray of the chest done. It it does not subside soon then get help of your Paediatrician. Hope this solves your problem very soon.Have a nice day.