I am doing my son's taxes and am an authorized representative. How can I use CRA auto fill?

You can use AutoFill with CRA represent a client, when you get to the Auto-Fill my return in Turbotax m this is where you need to connect to the CRA, stop and open up your browser. Access the CRA site through the represent a client option, and open up the account of the person that you represent. Keep that account open in the browser and go back to Turbotax and click on the import button. Turbotax will go to the CRA account that you have open in your browser and then download and autofill the information. 

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