I responded to the other thread you started prior to reading your post in this thread.
Given the fact that the initial transfer was motivated by his illness (that presumably could result in death), it could be argued that a gift was not the intent unless he passed during the time you were the remainderman. In other words, that the "gift" of the remainder was contingent upon his passing but not effective if he did not pass. A gift contingent upon an event occurring is NOT a completed gift and is not treated as such legally unless and until that event actually does occur.
Regardless of how this matter is treated, the only thing you need to be concerned with in the transfer is filing a 709 for the gift of the remainder back to the grantor (the son). If you haven't exceeded your lifetime exemption, you will owe no tax.
