He will not sell his merchandise to the player if they have positive reputation with the Strip. Equally, he offers a discount if the player has infamous reputation.
If reputation with the Strip is liked or higher, sometimes just standing or passing near him will cause him to spontaneously warn the player to stay away from him. If the player stays there, the warning will be repeated again and again.
Killing him will cause the player to accumulate negative reputation with the NCR, though not with the Strip itself.
If you attack Mister Holdout and chase him into the nearby gate leading to Freeside, he will run into the Securitrons guarding the entrance and be killed by them. As a result, his dismembered limbs will appear where he normally stood, which is inside the Strip.
If you have a Sneak skill of 50 or higher, you can convince him to sell bigger holdout guns.
He will sell you items for true value if you are shunned with the Strip. He will then also buy items for true value if you are hated or more.
Holdout is a term for concealing and hiding cards on one's body. Additionally, a holdout gun is another name used to refer to a pocket-pistol or derringer: handguns of very small size and caliber, making them easy to conceal, and meant to be used in emergencies or situations where conventional weapons are inaccessible.