He's probably bringing it up to you a lot as he trusts you and thinks of you as a close friend, not to just rub it in your face.

I know you're bound to be upset by it, and yeah maybe a little jealous, it's natural. I can bet you now a lot of people feel like this.

I know it sounds harsh, but really you've got to move on.
There's no point in being like "oh if he stayed, we would have... etc"
He's moved away, and moving forward.
You'll always be friends.

Don't feel idiotic for being upset, because you're totally not!

Try your best to move on, it'll be a lot easier for you.