Nausea and Prochlorper

My son age 13 has been sick for 12 days now. He had the terrible stomach flu and couldn't stop vomitting. His doctor perscribed Prochlorper for his nausea. He had a terrible reaction-his neck locked as far backward as it could, his body jerked as if he was having a seizure, he speech was minimal. Our Dr. never warned us of any side effects. Horrible! Horrible!