Greetings and we've considered downloading a number of number block apps.. but could you have to login/register with them also thus giving them your number and data, which we're not willing to do.. However, upon receiving a call yesterday.. I searched the options listed within my phone (the three dots on the right side of the screen when the caller's number is posted as missed call etc) while in the settings.. I noticed there is a "Report This Number as Spam" option, which not only reports this number or those number not on your contact list.. But blocks them as well. Soon after learning this, I've been using the "report" option more so than any number block app I've explored.
oh and by the way.. Please teach your son that his phone IS his phone and not to share it with any of his friends.. I suspect somewhere down the line.. he handed his phone to one of his friends, who now has his number and has possible passed it around..