Yes he did. He said it plainly in his inaugural address.
"I understand a proposed amendment to the Constitution—which amendment, however, I have not seen—has passed Congress, to the effect that the Federal Government shall never interfere with the domestic institutions of the States, including that of persons held to service ... holding such a provision to now be implied constitutional law, I have no objection to its being made express and irrevocable."
He plainly says that if the people, by virtue of their representatives in congress, enact such an amendment, HE HAS NO OBJECTION. That's not the same as an endorsement! That's saying he recognizes the will of the people.