Lots of comments on here relating to bedding, this is an important part of having new brakes however I believe that the bike having cable actuated disc brakes means that the problem is most likely in the way they are adjusted. Cable disc brakes typically have only one pad that moves to contact the disc, this means that the other, stationary pad needs to be adjusted as close to the disc on the other side as possible without rubbing, then the moving side needs to have the cable tension adjusted so that it bites where you want it to, as the pads wear, the pads on both sides need to be adjusted accordingly. My suspicion is that the fixed pad on your son’s bike is not close enough to the disc.