Hello, Thank you for posting your question at healthcare magic Sudden swelling and pain in testis can be due to these reasons 1. Epididymo-orchitis - caused by bacterial/viral infection of a tube at back of testis 2. Testicular torsion - Caused by twisting of testis 3 . Trauma - Unlikely in your son due to lack of history of trauma Epididymo-orchitis and testicular torsion is distinguished by physical examination Further investigations like urine microscopy and culture, urethral swab culture and Doppler scan to assess blood flow to testis may be warranted depending on the clinical assessment. Epididymo-orchitis treated with antibacterials,NSAIDs ,cold pack,testicular elevation and bed rest. Testicular torsion require immediate surgery to restore blood flow to testis Hope I answered your question Regards, Lekshmi