I didn't read the whole thread, but if PE some day became a cultural thing between males, in the case of a son, I would say it should start from the moment is pretty much born. In this scenario, professional medical companies should develop extenders specially designed for babies, kids, boys and on to be used in every transition of growth and development. Exercising the penis from a very early age will have no negative effects only positive. Just like the kid learn to control his bladder and pee in the toilet, he would grow up learning and understanding that the device has to be always with him, and eventually once he is able to follow instructions, learn about manual exercise routines and other devices.
That way in a couple of generations we could create elephantine creatures. PE on kids should not be limited to those who are born with a smaller penis, but rather to ALL males. Eventually some will grow up to a 13-15 inch dick, others to a humble 8 inches....but all would be benefited equally.