Holden tells Mrs. Morrow lies about her son Ernest--that, instead of being a big jerk as Holden say, he is well liked, he was nominated for class president, and, because he is so humble, declined the nomination and the position. Holden also tells her that his name is Rudolf Schmidt (who is actually the janitor in his dorm) and that he is on his way home early because he has to have an operation on his brain.
I don't think Holden is too sorry for his lies. He may be sorry on a basic level--he knows he's being dishonest, but Mrs. Morrow is so willing to believe everything Holden says, he just keeps going and going with the lies. It may also make him feel better to "get back at" Ernest for being such a jerk--at least to him.