Welcome to our forum.
Alas I can't think of a thread for your particular situation; I myself haven't explored the site that thoroughly yet. There is a "search" and a "tag search" function you can use (top of the page, a bit to the right), so maybe that would help?
Your best bet for making a mono/poly relationship doable, for you and this old friend you've been seeing, is to do some mutual learning about the details of polyamory and how it works, thus perhaps reducing the "scare factor" for him through familiarity. You might consider investing in the book "Opening Up
;" it covers a lot of ground and does it well. It might also help if your friend could join this site, become a member here and field the questions and concerns that he has.
Once you have attempted all that educating and familiarizing, if he is still dead-set against you being poly and is still devastated by it, then you'd probably have to break up (perhaps revert to being platonic friends if that's possible). Not a happy thought, but a reality that might have to be faced. You could only do the educating and familiarizing on the gamble that it *might*
work. If it doesn't seem worth the gamble, then I'd really have to suggest breaking up right away.
Would he be willing to learn more about polyamory? There's not much you can do about it if he's not willing.
Hope this helps in some small way.