In England some years ago there was a shortage of choirboys, many churches and even cathedrals started to allow girls to join their junior choirs. The sound produced by these mixed choirs was sweet but distinctly different to an all boy choir. As time went on the older boys voices broke but the girls were able to continue in the choir. The solo parts that go to the most experienced and proficient singers started to go to the older girls, while boys voices would break without ever trying the solo parts. Younger boys joining the choirs would find them dominated by older girls and finding no older boy role models would rapidly leave. Girls gradually dominated and took over most of the mixed choirs. Only the larger churches and cathedrals were able to separate and restart their boys and girls choirs.
The minority of younger boys in your nephew's school will find themselves dominated by girls the same age and older taking part in what are traditionally boy sports. As the boys reach puberty and begin to develop greater strength and speed they will overtake the girls who will drop out and get into girl sports and events.
To see how bad things could get you really should watch the 2nd tv series of Waterboys, start here with the original movie
http://www.dailymotion.com/video/x5z5do3