Yeah, I'm pretty confident he'd figure it out, and I actively encourage these things. He lives in NYC and takes all kinds of NYC transit and intercity transportation (planes, trains, buses--including FlixBus US!) by himself. But he's only been to Europe once and never traveled solo internationally, and mrsmicah gets nervous about these things. Also, he would be catching up with a group trip where the organizers are accustomed to very overprotective American parents and insurance consultants, so THEY might object to him arriving via public transit.

Anyway, it's looking like the whole thing will be moot because he's likely going to make the group flight for his trip and not have to catch up with the group late after all.

But thank you all for your input!