Its not about "telling him".
His father wants to give him some options to consider, rather than just following the crowd and the kid is open to the idea. Its about teaching him individualism and not to just blindly follow the crowd. Assess your options and make up your own mind - seems like a worthwhile lesson to me.
If he decides to go down the Beats route, that fine, at least he will have been aware of the alternatives.
In that case, make sure his dad shows him some of the more "accepted" headphone brands: Sennheiser, Beyerdynamics, Ultrasone, Sony, Audio-technica and so on.