He is not a romanceable companion. As such, nothing you say to him ever locks you out of future conversations or side stories/quests.
For companions with a romance option, you can be locked out of the romance if you turn down any
[Flirt] option that doesn't grant Dark Side points or others that are further along in the arc. To be safe, always choose all the flirt options if you want to romance a companion.
For the Sith Warrior there are two exceptions. First (major spoilers):
Jaesa Willsaam. If you want to turn her Dark side, or leave her Light side, you must choose the appropriate conversation options during and after fighting her master Nomen Karr.
Second (medium spoilers):
Vette. To progress her story, you have to remove her shock collar. If you do so too late, you might miss out on some earlier conversations — and the sudden change in your relationship is jarring.
Do note that this is a bit different in Knights of the Fallen Empire. BioWare has switched to a bit more Mass Effect-like system where it's not always clear what kind of impact your choices might have down the line. It's possible for your personal decisions to unexpectedly make a companion dislike you, which will prevent you from romancing them later.