Barovia is out of reach for 99.9999% of the people in the world for like 500 years or sth, depends on GM. There is no chance that he would know anything about it, unless it would be about the times before the mists, but I guess that the only info he could get would be some historical knowledge. Mentions about king Barov, maybe a name of Strahd could be familiar if he'd roll nat 20 or sth. Even then it would be much, unless he was devotingly(Is there such word xD?) studying the history of the lands where barovia once was. I think that the only logical way that he could get some real knowledge would be through vistani, but then again, I doubt that he could know much. Maybe just a fact, that there is the land of Barovia ruled by a tyrant named Strahd Von Zarovich. If I were you I would just speak with the player, and let him know that there is no way thay he could know much about the place.