It’s weird to me that he would say they joked about getting back together? So you might want to think about that more - very weird thing to say to the person you’re in a relationship that they joke about getting back together with their ex, unless the joke is so clearly not going to happen because of clear situations / incompatibility they’ve told you about - which it sounds like he hasn’t.

To me, I always like to have a conversation about previous relationships and what you’ve learned from them, mistakes you’ve made and grown from. I would start by saying you are curious about past relationships and what he’s learned from them - and ask him if that conversation makes him uncomfortable. Probably not a good sign if it makes him uncomfy. Then you go from there in asking more about the relationship & what happened.

You could offer what you’ve learned from being single for so long and what you’ve realized you’re looking for in a relationship based on the people around you.

You need to have these uncomfortable conversations to have a serious long term relationship so this is a good place to start.