It's awful when this happens, what sort of reviews and ratings did this host have I wonder. I am sorry your boyfriend is in this is situation late at night. Hopefully by now he's spoken to Airbnb and they are helping him.
It's a shame he turned up in a foreign country with no funds to cover any of his expenses, I wonder how he expected to live until he got paid?. You or a relative of his may need to wire him emergency funds to get a hotel or hostel for tonight while Airbnb help him find alternative accommodation if he has really turned up with nothing.
However accommodation is quite cheap in Bucharest so I really hope that he has enough for transport and a room for tonight.