If you were widowed in 2016 and your son lived with you, you can make a claim for the Eligible Dependant Amount credit for him. This claim is a Non Refundable Tax Credit and the credit amount in 2016 was $11,474.

https://turbotax.intuit.ca/tips/claiming-the-eligible-dependant-credit-5107

If you've already filed your 2016 taxes and did not make the claim, you will need to file an "adjustment" with CRA for the 2016 tax year. The attached information explains how to make the adjustment, but I suggest contacting CRA General Enquiries (1-800-959-8281) and they may be able to advise you of a different method which will expedite the adjustment process.

https://turbotax.community.intuit.ca/replies/2647992