resent ? You men angry ?
Then I guess it is against me. If it is about the city I razed, fair enough. It was a mistake and I should not have done it. If it is because change of alliance, well, that definitely is part of the game. If he thinks all the other people (except you of course) would just help him freely then give him their lands, then I think he prefers playing against AI. Less interesting IMHO but everybody has one's own preferences.
Finding a replacement ? Well, considering the few turns played, if he really dropped I would be more in favor of restarting, and with more humans (Spain, Ottomans).