Clinic Negligence

My sister sent our nephew to this clinic as he was sick for a week. They gave him medication including pills which were not suitable for his age and wrote a wrong dosage which led to him being hospitalized. His side effects were exactly the same as the ones indicated on the note found inside the box of the pills. He was admitted on Sunday and discharged on a Wednesday. Today the 12th of September was his first day from Baragwanath hospital. The school just called me to take him home as he is not feeling well. We are afraid this might have a long term effect on him and also disturb him academically because of their negligence. Please advice on how to take this matter further.