As someone who has a pretty ok relationship with my ex, I think you and this guy simply aren't in the right place to go any further. He has trust issues and you have a relationship with your ex that you are comfortable with. He can't handle that, and that's not to say that he is wrong or bad or anything, it just is what it is.

I have been through the whole online thing and yeah, it is tough. Oh, I have stories, lol. But when it comes to sharing the nature of my relationship with anyone I have dated, I tread carefully. If I were asked what you were asked, I'd say, yes, I have a good relationship with my ex, but I wouldn't share that he is my bestie. I think it can be off-putting even to the coolest of guys. I think to say it is "good" is enough at the very early stages where you are. If you are in sync in other ways and continue to get to know each other, then that comes out in due course.