My son died in 2018. I have done his return but i cannot find where i let CRA know he is deceased or where i can say this is a final tax return. Where do i input this?

I'm very sorry for your loss.  At the very beginning of starting the income tax return, you would indicate that you are preparing the return for someone else, enter the first name and it will ask if you are preparing the return for someone who has passed away.  Unfortunately, if you have already passed the interview section of the income tax return, there is no way to go back and change it.  You would have to start the return from the beginning.