I'm not sure of the circumstances of your relationship or why he changed, but one thing is for sure: you both have violated each other's trust. Don't know if you'll be able to fix that. Trust is hard to regain once it's been compromised.
It seems he was having some issues with you already if he was telling others he thought you were jealous.....but you're right, he shouldn't have sent your IMs to other people to read. All you can try to do is talk to him about it, and promise not to be snoopy anymore.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi