Make sure to check your son's personal information, like his name & address. Make sure the information matches what the CRA has on file for him. If you are not sure what they have on file, you can call and confirm it with them. 

If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are elsewhere, you can call 613-940-8495. This page has more ways to contact the CRA: Contact the Canada Revenue Agency

If he has a NAC (Netfile Access Code), try removing it; it is not mandatory. 

Also please make sure that all Mandatory Questions are answered. If he's not filing for the first time, make sure to answer No to the question "Is this your first tax return being filed with the CRA?".

If you are still not able to resolve this issue, please contact our phone support team for more assistance.