Much to the embarrassment of the President, Mary Todd Lincoln prevented Robert [Todd] Lincoln from joining the Union Army until shortly before the war's conclusion. On February 11, 1865 he was commissioned as an assistant adjutant general with the rank of captain and served in the last weeks of the American Civil War as part of General Ulysses S. Grant's immediate staff, a position which sharply minimized the likelihood that he would be involved in actual combat. He was present at Appomattox when Lee surrendered. He resigned his commission on June 12, 1865 and returned to civilian life.
http://en.wikipedia.org/wiki/Robert_Todd_Lincoln