Hmm... I have to be honest, but if I were Jim, I'd probably never forgive you. It's indeed a horrible prank. But let's pretend he's a nice and not a very vengeful man like me, because as you said he seems very nice. The only possible thing you can do is apologise, a lot, say that you're very, very sorry and that you will never be such a jerk with him. If he acepts your apologies, fine, then from now on try to stop these pranks and to be supportive and nice with him. Maybe he'll not forget, but maybe he'll forgive. But, as I said, this is very hard to take so give him time.