Double check that your son's name address & SIN number are the exactly same as what is shown on the document he received when he got a his SIN. If not, contact the CRA and verify with them what information they have on file for him.
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
