I just filed my return and I think I should have included my son as a dependant since he lived with me from Jan - Sept. 2015. He just graduated high school in 2015.

If he is under 18 and off to school. Yes you would include him as a dependent since he was with you for 9 months. Here is how you make any changes :

Yes, you can make changes (amendments) to your current tax return, as well as a tax return for any of the previous 10 tax years, whether you filed by mail or electronically using NETFILE. Here’s how you should proceed to make changes:

1. Wait until you receive your notice of assessment from the Canada Revenue Agency (CRA).
2. Once you’ve received your notice of assessment, visit the page http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/chngrtrn-eng.html?locale=en-ca&view=GBD_TURBOTAX?loca...
3. Follow the instructions on the CRA website. You can change your return online or by mail.
   If you choose to amend your return(s) online, the processing time is approximately 2 weeks.
   If you choose to amend your return(s) by mail, the processing time is approximately 8 weeks