Well if your talking about "Command" Haki then yes he has used it at least twice since the time skip in the manga, both times in the reunion arc and the first time with in Seconds of his first appearance...post time skip. He also uses the other two forms as well...the prediction one also with-in seconds of his first appearance back. The "shield" one to one shot a Pacifica. Also the number of people he knocks out is NOT a mark of him getting better at it...it is his ability to ONLY knock out those he wants to that marks his increase in ability. Remember at Marine Ford he knocked out a very large number of people as well.   See 10 other replies

Luffy has also used Haki prior to his arrival on Amazon Lily, canonically when he scares off Duval's (the guy from Sanji's wanted poster) water bison mount and against the guy who could change is body temperature in the filler (right before Thriller Bark, in the ice, with that guy named Phoenix who wasn't the Whitebeard one who saw that big ship then turned into a coward, and it was terrible). Also, Luffy actually uses Haki twice during the Whitebeard War. The obvious one is when he stops the execution but the other is when he predicts Hawkeye's attack would cut off his arms (since if he'd simply thought of that, we wouldn't have been shown an image of it happening without a thought bubble). This second use of Haki is the kind that was used in Skypiea, i.e. "mantra". It's also the kind that Coby was shown as using, though a different aspect of that element.   See 10 other replies