He can get away with this by Executive order:

§ 10. Modification of rules and customs by President
Any rule or custom pertaining to the display of the flag of the United States of America, set forth herein, may be altered, modified, or repealed, or additional rules with respect thereto may be prescribed, by the Commander in Chief of the Armed Forces of the United States, whenever he deems it to be appropriate or desirable; and any such alteration or additional rule shall be set forth in a proclamation.

Executive Order No. 10834 issued by President Dwight D. Eisenhower on August 24, 1959, amended the provisions of Title 4, U.S.C., Chapter 1 and established the 50 star Flag as the official Flag of the United States, effective on July 4, 1960.