My 19 year old son is complaining about his right side abdomen feeling tender and painful when pressing on it or having a bowel movement.

Should see a doctor: He should see a doctor if he has right side abdominal pain to make sure it is not appendicitis or other serious abdominal condition.

Multiple possibiliti: Several different possibilities exist including gallbladder problems, constipation, hernias, or small intestinal problems. Less likely serious colon issues. Bottom line is that he should get checked out by a gastroenterologist and get a good physical exam and history taken so appropriate testing can take place.