Hello! Pain and swelling in inguinal area (or better pain and some kind of lump) may be caused by inguinal hernia. Especially when it hurts during coughing. In inguinal area there are also lymph nodes which can be painful and enlarged when inflammation process is in this lymph nodes area. It can be located on same side leg for example. If your son is having a fever this diagnose is more probable. One more thing - by inguinal region I understand a region where an abdomen ends and a leg begins. If the pain is a little bit higher, there is a need to look for a disease in abdominal cavity. If it`s a right side where the surgery was performed, intra abdominal adhesions are possible, and intestine partial obstruction must be ruled out. Also there is always a probability of post-operative hernia. What should be done is: blood tests for inflammation signs, ultrasound of abdomen and inguinal area and if lymph nodes will turn out enlarged, biopsy is necessary. Take care