Yeah, he needs to cool his jets. Don't press him.
He may be just acting upset because he already knew you had feelings for her, and hoped you'd never say anything.
If y'all are all close, it's tough. You've probably been giving out those signals by accident to dude and he just suppressed it without thinking.
My little sister is dating one of my close friends, and I don't trip because I know his heart is in the right place. I know this ain't siccness talk per se, but that's my little sister and I'd die for her.
I've run off a few sorry assholes she didn't clue in on their intentions. You just gotta be firm and stand your ground, but respect the dude for getting caught up in the moment.
He's probably thinking of all the dirt you did and told him about, too, so that could just anger him until he lets it go.
If the chick really means a lot to you, you've got to be delicate about the whole process on both sides, his and hers.