Hi K.,
Wanting to let him know that YOU KNOW about his stealing in the past....I would feel the same way BUT...... just because you KNOW about his stealing, doesn't mean you should address the issue. I think being "aware" of the issue is enough, especially if he doesn't do it to harm someone (sounds like it's for other issue's).
Anyway, if it were my situation, I wouldn't say anything to him, but I would tell my grown children to watch their stuff because he has a problem in the past.
You and your girlfriend are just recently reunited, which means that long distance talking/emails has been your friendship. Sometimes, we women, reach out to our more distant friends when a "family problem" arise because we don't want to talk about it with anyone locally (embarrassing, or whatever.....). So, for now, I would keep that stealing information between you and your friend.
That's just my opinion.....but watch him like a hawk!
~N. :O)