Pardon my brashness, but you have gone too far with this joke. First of all, when you make someone pretend to be another person's girlfriend (or boyfriend), it's asking for trouble, whether how hilarious someone else sees it. You never prank a friend making them believe they are in love with a pretend girlfriend, because you don't know how their state of mind or their situation with love is.
You know it's bad when not even his parents can get him able to talk to you. And this isn't the kind of things that can be solved with a "Chill, it's just a prank" to just wash out the consequences. It might've been a poorly explored idea if you thought how loud you were gonna laugh at it instead of thinking of the what-ifs. Get in his shoes. If he had done the same thing to you... if he contacted your crush and convinced her to pretend to like you, just for you to wildly fall in love with her and then she tells you it was all a ruse in front of the school... how would you feel?
Also, believing he won't retaliate to your pranks just because he's smaller than you two is quite disgusting.
I get why he doesn't want to talk to you. I mean, I have a brother and a cousin who teased me and pranked me a long time ago. All I can say is let time heal the wounds and then talk to him. And most importantly: be honest. There's nothing worse than an apology that sounds forced and dishonest.
Edit: I know you mentioned it isn't, but if this is a joke thread, you're more scum than what you mentioned on the OP.