But if he is a liar, then saying he's a liar is also a lie. He also can't be telling the truth or saying it would be a lie. Therefore the only truth is he's is an enigma.

Or the fact that no one ever tells the truth or lies all the time. Note that I said that so it's not true all the time.

But if it is true all of the time, then you sir, are a liar and cannot be trusted. Him, on the other hand, can also be called a liar, making you tell the truth, which makes him a liar, but if you are a liar, he is a truth teller...

Logistical Paradoxes for the win...