Fever in infant

Hi mommies! My son just went for his monthly jab. He has fever 39.3. Must we go to kkh once if his temp rises to 40? I’m so worried. We just gave him milk. His usual feed is 120 ml every 3 hrs but we reduced it to 90 ml as he cannot finish his last 2 feeds. Right nw he left 50 ml. He drank 40 ml only.