Please explain why does my child always vomit when he gets a fever?

Vomit with fever: When the body is fighting illness, it considers gut function to not be essential, so things slow down a lot. That's why you usually don't feel like eating when you are ill. Some peoples bodies take this to the extreme and almost shut down gut function, which leads to vomiting. Try just sips of fluids when he is ill or has fever and see if he can tolerate this.