I am looking for a word to let others understand that somebody is a person who does exactly the opposite of what he speaks. Is there any word for this?
A dissembler is a person who makes false appearance. One of Wordnik's sources has this usage note:
Synonyms Dissembler, Hypocrite. A dissembler is one who tries to conceal what he is; a hypocrite, one who tries to make himself appear to be what he is not, especially to seem better than he is.
So which word would be the better of those two would depend on how much the individuals in question are actively portraying themselves to be someone they are not. In other words, are they merely hiding their true nature? Or actively pretending to be the opposite of how they really are?
This sounds like a contradictory person:
asserting the contrary or opposite; contradicting; inconsistent; logically opposite: contradictory statements.
Hypocrite would be a good word if the man in question is malicious. But you didn't indicate if this man is exhibiting this behavior in a morally-neutral sense or not.