Most Helpful Guy
Most Helpful Girl
Let me tell you, what you're going to do.
You are going to sit him down, cook a nice meal for him, treat him like the "prince" he is, and then you are going to say something along these lines:
"Baby, I just want you to know that I love you, and that I appreciate you for the friend, the lover, and the man you are to me. I think you are an amazing person blabla (sugar coat it basically at this point). I really miss what we had, and I can see that you care about me, and you need to know that i see the effort you're putting to make it work and that i care for you and our relationship very much. And because it is you, because it is our relationship, i have thought about it for a long time. And i am sorry, i may be disappointing you, but i am not mature enough to let you be close to this girl. It is only my issue, and i am sorry that i cannot deal with it on my own, i really wish i could, but i cannot find it in myself at this point of my life and of our relationship, to not hurt when i see you contacting her. Because of how great you are, i need to let you know that and again I am sorry, i wish it could be different. I have too much respect for you, to pretend that I don't care. Because if I do pretend, this relationship is not going to work, because of me."
NOW, let me be clear it do not believe a single word i just wrote, but from my own experience this is the only thing that works. Men don't respond to threats, yelling or begging. But if you tell him how important he is and blablabla you may just get want you wanted.
Also, if after this, his response is not "ok i am going to break it off with her", let him go and move on. Because no matter if this relationship with his ex is purely friendly or there is something there, if you feel uncomfortable with it, that's just it, he needs to accept it, and work with it.