Is your son aware that all Xboxes experiencing the ring of death have had their warranties extended for three years beyond the purchase date?
This is due to poor manufacturing processses such as lead free solder which is more brittle and tends to crack easily. Also the heat sink clamp can cause the motherboard to distort when it heats up, again causing solders to break.
Other than that it could be caused by a surge proter if he's using one. They can go a bit flaky and not provide enough juice, so try plugging it directly into the socket.
Or try disconnecting the hard drive ( after unplugging). Now plug back in and turn the console back on. If console does not show the three red lights error, turn it off and disconnect the power cord from the wall. Finally, reconnect the hard drive, plug in the power, and turn on the console.
If it still shows the rings of death again then you may be able to just replace the hard drive.
A build up off dust and/or overheating could be another cause.