That was a really cruel prank there. Borderline harassment if you ask me. People who have never had anyone to love romantically know the pain of lonliness all too well. The fact that you did it as a prank will make him feel absolutely worthless. If he was dealing with any sort of mild depression beforehand, this event could cause him to fall deep into depression and in even severe cases suicidal. I'm just going off of my own judgment here because if this happened to me I know I would never be able to get over it.
The only thing I can recommend is to find a way to make it up to him. Also don't be too invasive and give him the time he needs to be alone when he needs it. I hope he gets better. Good luck.
When i was in high school a situation similar to this caused a kid, Eric Johnson, to kill himself. His best friend played a prank on him, making him think he was messing around his girlfriend, before they could tell him it was prank he stormed off. He ended up driving into the mountains, shot himself in head as he drove his truck off the side of the mountain. These kind of situations can push people over the edge.