Your son would have to do an adjustment to his 2016 income tax return to report the income from the RESP. This can be done online if he has a My Account with CRA or can be done on paper and mailed in to CRA. I have included a link with the T1ADJ form for your information.
https://www.canada.ca/en/revenue-agency/services/forms-publications/forms/t1-adj.html
