Hmm, it doesn't sound like the communication has been good. So, if I've got this right, you approached him during your relationship wanting to have an open relationship. He avoided the topic. Recently, he met some one else and wants to break up. N'est pas? He sounds like he's not trying to be all that considerate of your feelings, poly or no... As to whether this is a pattern, I would suggest looking at how his relationships have ended. Perhaps he's an NRE junkie.
I don't blame you for feeling ambushed. I think you would want to take a long hard look, talk to some friends who know you both before you would consider continuing anything with this guy.