Suggest Remedy For Diarrhea Following Antibiotic Course

Hi. My 10 year old son had a rash on his elbows and knees, red in color and started spreading over his arms and legs as well as back and stomach. His Dr was not able to diagnose the rash but gave him an antibiotic assuming it was a bacteria rash. That was 2 weeks ago. He took the RX for about 5 days. After the 2nd day he started having diarrhea. I was afraid the RX was the cause of his diarrhea so i stopped giving it to him. His rash has now disapeared. The diarrhea has now lasted 15days. He is loosing weight. Looks very frail and very pale. He can not keep anything, not even liquid in his system. Should I have his stools tested? I am becoming very concerned! Thank you