For both 2014 & 2015, you will have to mail in your returns. You can find the address to mail them to on this Canada Revenue Agency (CRA) page - Where to mail your paper T1 return
Phone CRA to get yourself set up as an authorized representative for your son and request a Represent a Client account for yourself to log into his MyAccount Online with CRA. You will find Tslips issued to your son for those years and it will identify which returns have been filed already, or not. Then, you can submit the adjustments online as well.
@lusciousconcepts Unfortunately, we do not have software available for 2014 to 2017. You can purchase TY 2018 directly from our website. You might have to consult an accountant to help file prior year taxes.