Technically, no sitting President has ever fired an Attorney General they nominated to office with Senate approval. But President Trump clearly has the power to remove Sessions, based on the Constitution and past legal decisions. And most importantly, he can ask for his resignation.
The Attorney General of the United States – appointed by the President and confirmed by the Senate – heads the DOJ with its more than 100,000 attorneys, special agents, and other staff. It represents the United States in federal criminal and civil litigation, and provides legal advice to the President and Cabinet.
United States Attorneys are appointed by the President of the United States, with the advice and consent of the United States Senate, and serve at the direction of the Attorney General.
However, the following U.S. Supreme Court cases clarified the president's sole removal authority: Myers v. United States (1926): The court held that the power to remove appointed officials, with the exception of federal judges, rests solely with the president and does not require congressional approval.