I too, had a “past”. I don’t want my chosson to know, and I probably won’t tell him everything after marriage. Nevertheless, I don’t think it’ll harm our sholom bayis. Those things are literally my “past” and I let it stay there. I live in the present and in the future, so I won’t return there.
I think you have to find out where this guy is up to. Perhaps have a talk with him. See where he’s headed to and what his goal is in life. If you see that it is indeed bad and not suited for the girl, ask a shaila.