Dangerous PDA watching while moving

Watching Personal Display Assistant while walking is very dangerous especially in crowded circumstance and it is called attention.
In addition, wearing earphone at the same time invites much danger, because the 2 senses of 5 ones in human being are not in function scattered for another concentrations. Not while walking but while riding bicycle or driving a vehicle, the degree of danger should be far increased at all.
In spite of such an information, my son does it while riding bicycle apologizing with special designing of earphones, it can catch also external sound.
What kind of way is remained to change his perception in this dangerous  situation, bad boy, how come?