My Son Is 23 Year Old. A Week Ago He

my son is 23 year old. A week ago he had one episode of diarrhea vomiting and dizziness after his lunch. He was ok after a few hours. Yesterday it happened again, diarrhea and vomiting. It lasts a whole day. He now feel weak and tired. I can say it isnt food poisoning because nobody else got sick after eating the same meal. Could you tell me what could be the cause? Thank you.