All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is not permissible for you to give your phone number to that boy as this is a means that leads to prohibitions. Even if the purpose of this is nothing but good, then the devil may lead both of you to what Allaah has forbidden. Allaah Says (what means): {O, you who believe! Follow not the footsteps of the devil. And whosoever follows the footsteps of the devil, then, verily he commands Al-Fahsha' [i.e. to commit indecency (illegal sexual intercourse, etc.)] and Al-Munkar [disbelief and polytheism (i.e. to do evil and wicked deeds, to speak or to do what is forbidden in Islam, etc.)}[Quran 24:21]
Indeed, the Sharee’ah came with the rule of blocking the means to evil, and this rule is one of the clearest pieces of evidence about the wisdom of the Islamic legislation and that it was revealed from Allaah, The All-Wise, The All-Aware. For more benefit, please refer to Fatwa 81356.
In addition to this, this matter that is intended, which is convincing that boy to wake up for the Fajr prayer, can be achieved by any of his male friends doing that.