Personally, this is one of the scenarios I hate most. The fact that it is a freeroll means a lot players will probably have a tendency towards loose play. Add to that the fact there are only two active players, while the rest are sitting out. The name of the game becomes "Steal the Blinds", and in this game aggression often wins. To assume that your opponent is automatically a donk is the first mistake. He was likely excercising a very specific tactic againts you, raising just about every hand, betting out on every street, putting the pressure on you. Obviously, if he will play any two cards, then it's hard to know whether or not he has paired even on the most random board. Even his refusal to fold is most likely a calculated risk, to discourage you from trying to bluff him, allowing him to be the aggressor. Just because you don't realize it, doesn't mean that he doesn't know exactly what he is doing. And with only you and him at the table, he's likely to be watching you as closely as you are watching him, looking for patterns, trying to get a read.
Or maybe, he did just make a horrible call and was lucky as hell. Believe what you want, or learn what you can. The choice is yours.