Man here, I think I can understand his fear. While there's no need for fear in relationships as we all know, it's still something that is worth knowing to have a better understanding of each other, instead of denying those feelings.
This may seem intensive and crazy, but we're talking about a fear mechanism here, so here it goes.
The past got triggered in him because of the STD from the past.
STD can correlate to irresponsibility, casual sex life, and in extreme internal reactions, correlations to prostitution. On the long view, irresponsibility, casual sex life, prostitution, etc, can correlate to commitment issues, loyalty issues, etc...
It seems crazy, but this is the subconscious mindset of fear of abandonment.
I'm not saying you have commitment issues, loyalty issues, not at all!
I'm saying that this is what the fear is saying.

I'll tell you a personal story about my "numbers".
In my third relationship, my partner actually started with this. She asked me if I ever had a relationship, why we broke up, etc...
I mean, I don't really mind answering, but thinking about it right now, this was an anxious thing to ask. But it was mutual, as I asked her afterwards about her numbers, too, but it was more for mutuality rather than really caring, as she'd ask about me more than I'd ask about her in general.
Well this is my story, I kinda felt like sharing it with you

I think a good thing to do is being compassionate, giving a bit of place for fear instead of seeing it as fully wrong, because fear is also part of our nature, even among ourselves, for whatever reason there is as there are many (mother abandonment, attachment, jealousy...). How about a loving conversation about how you can go further forward in your relationship?