Very strange, but actually I think this same thing happened to some Chinese mantises that hatched early this Spring. Some of the nymphs would just be on the bottom of the cage with their legs up in the air, or something and almost as if they were paralyzed or too weak to move. It was very weird. I think it was possibly poor genes or they didn't develop right or maybe they were contaminated with some kind of poison (which is very doubtful it was poison). Its possible your case is different and it could be some kind of bacterial infection has infected your little guy. And sadly I am not certain your little guy is able to be saved, but I wouldn't give up hope on him. Try feeding honey like Conner said.