He could be a son of Derek and Mark's sister?
Or the son of Nancy/Kathleen/Lizzie and Mark who was given up for adoption? Nancy implied she slept with him.
In light of what's happened recently and the fact they teased at Derek having a secret son in 10x13, it would make sense if it's his long-lost son. Would stir up drama for Meredith, that's for sure... Also corroborating Jesse saying his present looms large.
