According to a few trustworthy lexicon services, a "yes-man" is...
(n)- A person of unquestioning obedience; a follower- a person who accepts the leadership of another.
And well stated by Mirriam Webster Online—a person who agrees with everything that is said; especially: one who endorses or supports without criticism every opinion or proposal of an associate or superior.
HERE IS YOUR ANTONYM!
