So, yeah, I asked my lawyer friend. A durable power of attorney will grant me legal authority to sign my son's tax returns. Hopefully this is useful to anyone else who has a similar question.
However, as previously stated, TurboTax apparently does not allow you to remove the "SELF PREPARED" stamp on the preparers line, which there should be an exception for because you might have to do taxes for a family member who is either absent (such as military deployment) or medically incapacitated in some way.
