I did see a loophole:
(11) An only son, after quarrelling with his mother and father, comes [to the sangha, saying], "Let me go forth." Upon being told, "Come back after you have obtained leave," he says, "I'm not going! If you don't let me go forth, I shall burn down your monastery, or stab you with a sword, or cause loss to your relatives and supporters by cutting down the plants in their gardens, or kill myself by jumping from a tree, or join a gang of robbers, or go to another country!"
It is suitable to let him go forth in order to safeguard life. If his mother and father then come and say, "Why did you let our son go forth?" they should be informed of the reason for it, saying, "We let him go forth in order to safeguard life. You may confirm this with your son."