I think we’re talking about two different things here, which are easily confused because they’re both called “being friends.”
First one, there are long-term friends we have through our lives. These are the people that are there for you, to listen and talk to, that catch you when you fall, that you can rely on to be there, and with whom you have made important social and emotional connections. Some of these, for me at least, are ex-boyfriends. I mean, after all, we started out together because there was a lot in common there, we had a long bonding experience, and after all the fireworks and explosions of the breakup were over, we still connected as human beings, even if it took a few years (or decade) of not speaking. Some of these ex-boyfriends even came to my wedding. They are my true friends.
However, it’s easy to confuse that with “Justbe” friends. These are sometimes relationships that have just broken and haven’t had a chance to cool, and where emotions and hurt are still running high. It’s tempting to be Justbe-friends with guys like that because you hope they’ll “see the light” and get back with you, or perhaps have ex-sex, but this is a recipe (usually) for disaster and hurt feelings. This is still a relationship (albeit a broken one), not really a friendship yet, even if people call it that because there’s not really a word for this in English.
Here’s a good way to tell the difference: how would you feel if this man told you he’d found the light and love of his life, and that he was going to marry her? Does it cause a stab of jealousy or hurt in you? Then you are not quite friends yet. Does it, however, make you happy for him, delighted to meet the lucky woman, and would you go celebrate their wedding with them and be genuinely happy to be there? Be honest. If you can say yes to this, he is probably a true friend.
It’s not impossible to turn a Justbe-friend into a true friend, but it takes time. Sometimes it takes years. It’s not something that can be forced, and it works on its own schedule. But it does happen. You say, ” I don’t know many people at all who are good friends with their ex-partners.” How old are you? I ask, because it often takes years for this to happen, and if you are young (say, college age, or under 25–30), there might not have been enough time for this to naturally happen.
If there’s any doubt about this guy, I’d say to just chill out for a while. Let some time go by. There has to be a certain amount of letting-go for it to work, if you really do end up as friends someday. I know it sounds like a long time now, but just live your lives for a while, and if you two become friends later in life, then you will. And if you won’t, you won’t. It’s sort of a zen thing. :)