This has been going on for two years correct? Is the problem that he is talking to his ex, or that he is lying to you? If it is just the lies, then tell him that. Do you trust him not to cheat on you with her? There is the possibility that they are just catching up and trying to be friends again. Maybe you already know what the real problem is, but if not, find out what really bothers you more. Do you want him to be honest with you while still talking to his ex? Or, do you want him to stop talking to her all together? He is lying to you because he feels like he is doing something wrong. It is possible that he is doing something wrong, but if they are just catching up, then what is the problem? Both of you need to find the root of the problem, and then go from there. Why is he really lying to you, and what hurts you more, the lies, or him talking to his ex?