He probably can't talk (Zombie check: Depends on the state and area of brain decay, as well as decay to the vocal organs.)
He can't move (Zombie check: That would mean either some serious stiffening post-mortem, or loss of all forms of locomotion, or decay to the point of no locomotion.)
He will automatically rot away after a while of being no buried (Zombie check: Theoretically, they should all decompose still. Just because they're undead doesn't mean they're immortal. Of course, I don't recall any zombie movies in which the ultimate outcome was waiting for the zombies simply to decompose naturally.)
So...the signs show that he could plausibly be a zombie...but probably is not.
