Hi, I Am Unsure Of What To Do With My

Hi, I am unsure of what to do with my son. He is 2 years old. He started developing a fever on Sunday. And then again around noon on Monday. Tuesday he was fine up until about 6pm. He then proceeded to violently vomit and develop diarrhea. We took him to the ER and they diagnosed him with a stomach bug. They also checked his urine by collecting a sample through a catheter. That was around 11:30 pm. Come Wednesday afternoon he still had not urinated. I again took him to ER around 5pm. They did a bladder scan and saw he had 200cc's. Come around 8:30 pm he finally urinated. It was extremely painful for him. They sent us home. Today is Thursday and he has not eaten anything in 3 days is barely drinking if anything if at all, and he has just been going in and out of sleep for the past 2 days. He is extremely weak. Should I take him back to the ER?