What you are looking for is a life estate
A "life estate" is an estate whose duration is limited to the life of an individual (usually the party holding the life estate), and a legal arrangement whereby the "life tenant" during his or her life retains use (the rights to rents and profits), possession
of the property and costs of maintaining the property. The life tenant cannot sell or waste the property without the consent of the "remaindermen".
How is a Life Estate Created?
A legal life estate in real property can be created by conveying the property by a deed which carves out the life estate for the grantor and creates a "remainder interest" by which the "remaindermen" (this would be her son) receive full ownership
(fee simple) immediately upon the death of the life tenant (grantor).
Here is a sample: http://www.giveawaylegalforms.com/glf/directory/deeds/deed-for-life-estate.php
You can also visit www.legalforms.com and get one there but given the situation you should have this drawn up by an attorney because I am sure the minute she is gone, the Son will come after the house and challenge you so be smart and have an attorney do this for you.
As to TN common law: In the State of Tennessee it doesn't recognize common law marriage, but Tennessee will recognize common law if the couple lived together long enough as common law man and wife in another State which did recognize common law and have made Tennessee their home.
I hope this helps. Please click "accept" so that I receive credit for answering your question. I will be happy to reply to a follow up on this issue at no extra cost. Your Positive feedback is appreciated. If you want to request me in the future type "TO GEORGETOWNLAWYR" in the subject line and if I am not online I will answer asap. Thanks and best regards.