Il give you another perspective. I have dealt with something called retroactive jealousy. I don't choose to think that way, I don't want to at all, but it is a form of obsessive thinking and it's PAINFUL.
When you first tried to bring up the subject of past partners, (something I truly don't understand) he maybe didn't want to talk about it because he knew he couldnt handle it or had the jealousy issue.
When it was forced into the mind, obsessive thinking and acting comes into play. E.g checking your email history. Now he is faced with the vivid detail in his mind and it's painful. The more he tries to stop thinking about it, the more it comes to mind. Rationally, he may know that the past doesn't matter but that doesn't stop the thoughts playing through his mind.
Its a horrible issue to have, iv struggled with it for as long as I can remember.
My situation was a little different, but was triggered in a very similar way. I was upfront about it to my partner, i told him i struggled with it and that i was wanting to overcome it. My partner was unfaithful and untruthful and although I hated doing it, i dug into his past and seeing how he was before I met him, in a way shattered the way i thought about him. I had details, pictures and made up visions about his past that made me very insecure.

It's something that he will have to want to overcome himself. Im in therapy for it and my therapist told me it works the same way OCD does. It's a struggle and I realize my partner didnt deserve to have his past brought up and that it shouldn't matter. It's probably best that you sit him down and discuss it further and see if he's willing to work through it fairly, or let you go.
All the best