What Is Epididymo-Orchitis?

Hi Dr,2 days bk my 8yr old son suddenly complained of severy stomach ache and vomited..i gave him emset nd he slept that night. next day his pain wasn't there so i sent him to school, after coming back he complained his painful nd swollen left testis. I took him to the Dr. in the evening and the Dr. suggested immediate ultrasound also prescribed augmentine for 6 days.yesterday got his ultrasound report and it shows significant hydrocele in left scrotum and mildly increased vascularity in left testis Suggestive of mild left Epididymo-Orchitis. Please help me understand the cause of all this as it suddenly happened and suggest what should be done.