Ugh. First of all, congratulations on taking this step. It sounds like it was a long time coming and something you needed to do. Second, my suggestion is to lay some very firm boundaries ASAP. The biggest boundary being that you need him to stop contacting you in any manner. You can be loving and kind but you need to be firm. Something akin to "From today forward, I will not answer any of your communications and you must stop contacting me. I care about you and am very sorry that you are hurting, but I am not the person to help you through this."

Then you must be true to your word. If you are truly concerned about his mental health, then make contact with someone in his life that you know he can lean on and let them know that your ex needs their help. Make it clear to that person that you, however, need to cut off all contact.

I had a very good friend whose on again/off again boyfriend of seven years did just what your ex is doing every time they broke up (which was somewhat often). He would basically stalk her until she relented. This went on for a very long time (and she lost me as a friend in the process) because she refused to lay firm boundaries and stick to them. It was so manipulative, on both ends. There are those couples that can break up and remain friends/in contact with one another without having to cut one another totally off. I'm guessing your relationship isn't going to be one of them.

I know it may feel harsh, but honestly, it's in the best interests of both of you that you cut off contact. You are simply not the person that is going to help him heal. It doesn't often work that way.

If you need a bit of fortification, read the Dance of Anger. It's not completely on point, but it offers some very good suggestions on how to lay firm boundaries with people. I learned a lot from it in this regard.

Good luck.