This isn't your issue, its his. I've talked about past relationships with guys, but only in the context of them wanting to know what I've done and what I'm up for (haha, yes, it turned him on to hear about my sexual past). But no, he wasn't the jealous type, he knew I was only into him, and he didn't feel insecure in the least. Your guy is probing because he feels insecure. He is tearing you down in order to make himself feel better. I'd say let this one go.