I think he'd have to be. After all, Cain lived for millennia with the Mark. He also has some of the can't-be-killed sense of immortality, because if he is killed he comes back as a demon.
I haven't seen seasons 10 oder 11 yet, but my guess would be either a special weapon oder spell, oder Cain isn't really dead. oder are Du talking about Dean at the end of season 9? He wasn't fully immortal in the sense of being impossible to kill, but he could easily return as a demon, as we saw.
