Unfortunately, you need to either create a separate return for him in your account or have him create a TurboTax account. As per CRA, if he has no mental or physical infirmity, he is no longer your dependent, hence cannot be added to your joint return. Moreover, If your son is in University/College he would need to do his own tax return to get his tuition credit.
Please note that, on an account you can create multiples returns for different people .
Thank you for choosing TurboTax.