I suspect that based on studies that show jogging and biking sway pink, the anecdotal evidence from the weightlifting sites (but this ~may~ be due to supplements that they are taking and NOT weightlifting per se; we've also had some natural weightlifters on the site who did not take supplements and had all boys), and the fact that we've had several professional athletes (well, their wives!) among the people who bought custom sway plans. A pro hockey player, a pro BMX biker (actually TWO of these guys believe it or not), a pro rugby player, and a pro American football player and some other very serious athletes as well they all got blue plans. Small sample size but it's interesting.
All that having been said we do see among pro athletes in the public eye, both boys and girls being born so this is in no way a magic bullet but just something to be aware of.
If he won't do it, he won't do it and we have to take care not to ask him to do too much. :)