It definitely is scary when a partner comes out as poly (or, in this case, poly-curious it seems), and the distance would make it worse.
You are right that the unknown is the worst part. When you can know and meet people, and feel part of the relationships, it's much easier, not that it's necessarily easy either of course.
As you going to be long distance for a long time, do you know? I would sincerely recommend putting this off until you can be physically together. Both because this way you can be sure you're not "filling a void" caused by not being together (which wouldn't be nice for the partners filling these voids, nor good for your relationship I think), and so you can be there to walk through it together.
Alternately, you could go at it if you talked a lot so the other feels involved. The problem is that just hearing about people and not actually getting to know them is probably not going to work. Your imagination fills in the blanks in the worst way possible. It would need to be people you get to talk to as well (online or something), and that before he has a relationship with them. So you can get to know them and see how comfortable you feel.