The simple fact that he is hiding something from you as significant as financially supporting his ex should be sufficient reason for you do leave this relationship. Also, most people who are lying about something or trying to hide something will, when confronted, admit to some smaller sin to try to keep something even worse hidden. To spell it out, if he is admitting he is helping her $$$, then that may be just the tip of the iceberg; he may still have significant feelings for her or something even more devastating. As a former, recovered semi-professional liar, I speak from experience. Good luck.