My 12 ear old son was doing exercise and now his lower abdomen is very sore, twisting pain, it looks slightly swollen and he is in pain what should i do ? help please

Postexercise pain: If he is having very severe and persistent abdominal pain he should be examined in the hospital emergency room where prompt care is available. A simple strain of abdominal muscles wouldn't cause abdominal distension. Testicular pain would be quite serious. An inguinal hernia would result in a groin lump. If he's not nauseated you could administer some acetaminophen in the meantime.