The boy standing on the roadway in front of the north façade is Lincoln’s youngest son, Thomas “Tad” Lincoln (1853-1871). Soldiers appear in the background and may well be members of Company K, the 150th Pennsylvania Volunteers, also known as “Bucktails.” The Bucktails, so nicknamed because of deer tails worn on their hats, provided protection for the president and his family during the war. Tad was popular with the soldiers, and as a memento, the Bucktails presented him with an album containing their autographed photographs.
Late in the war, photographer Henry F. Warren from Waltham, Massachusetts, hoped to photograph the president but had no means of gaining access to him. A chance meeting with Tad, however, enabled Warren to establish contact using the boy as an intermediary. On March 6, 1865, the busy president agreed to pose on the South Portico, and three images were taken. These are believed to be the last photographs taken of him.
For more, please see the complete White House History Journal article: "Photographs of the Lincoln White House" [PDF] by Lydia Tederick