My 19 Year Old Son Started Running 2 To 3

My 19 year old son started running 2 to 3 miles daily about 40 days ago. He has sharp stabbing pain now under his ribs on his right side when he walks long distances or tries to run. He is an avid tennis player and this does not bother him when playing tennis or when he breathes. He is a college student and does need to walk long distances to class. Advice? Thoughts? My email is YYYY@YYYY . Thank you!!