A few things that can cause a complete rear brake fail:
Brake not properly bled. Air in the brake line expands as it is heated up by the friction of the disk to the rotor. Also note, the line runs parallel and next to the exhaust. Air expands with heat fluid does not (for all intent and purpose here). The expanding air then displaces the fluid back to the reservoir. Then when the rider applies the rear brake, there's not enough fluid in the line to activate the brake.
This can also happen with a ABS equipped bike, when the bi-annual fluid flush has been overlooked.
Brake fluid can draw moisture from the atmosphere into the brake fluid. Over time it becomes oversaturated with water combined with the brake fluid. This is why the service manuals say to flush the brakes every 2 years. (I prefer to do it annually) Brake fluid gets hot. Brake fluid has a high boiling point. While water has a low boiling point. The fluid gets hot, the water turns to steam, this increases the pressure in the brake line and acts the same as the rider applying the rear brake. Thus increased drag / more heat.... You get it...
Then of course the obvious; Hydraulic leak, Not enough fluid in the reservoir.
A 13 year old bike should have had the brakes flushed 6 times by now... Did you get the service records with the bike?
Hopefully your son made it home safe!
I hope this helps you... Please post your findings.
EDIT: I just noticed the "NEW BACK BRAKE" at the end of your post.
New brake? Rotor? Pads? or the whole kaboodle?
And was this new brake DIY or from a M/C shop?