Yeah, you've run into the hopeless hope case.
I would say that he still has pretty strong feelings for you, and he hasn't been able to get rid of them. Someone in that situation will interpret everything, no matter how tentative, as a hopeful sign that you're returning their interest. Saying "If I ever want a relationship, I will tell you" doesn't mean, to them, "No, never, back off", it means "Yes, there's a chance she'll decide she wants a relationship! Woo Hoo! If I'm just patient enough, my time will come."
So in a situation like that you have to be blunt. "No, I'm not interested in a romantic relationship. I will never be interested in a relationship, we've tried it, and it just isn't there."
At the point when it become apparent that he still is holding out hope, the kindest thing you can do is back off. Either tell him "Look, I know you still want a romantic relationship. I don't, and keeping it to friendship isn't working. I think we need to back off.", or just quietly withdraw from the friendship.
But basically, the *only* way a platonic friendship after a romantic relationship works is if/when *both* of the people are completely over the relationship. You decided that you're not interested in him and broke of the romantic part. He's never gotten over his feelings for you and wants the romantic relationship back. Keeping contact will mean that he isn't getting the chance to get over things.
In other words, I think your friendship is over, at least for the next few years.