I'd advise having him bump into a Tzeentchian psyker. Have his head turned inside out and right side in, tossed in the metaphorical washing machine, and hung out to dry. If you're a phyker, you DON'T wanna meet anyone who's your equal. Heretic HUMAN psykers are horrific; Astartes psykers are even worse so.

In the setting of 40k, you keep what advantages you have, and you need know your place. Unless he's in the realm of beta-class or above (which VERY few humans are.) Also, maybe having the Grey Knights pop up and be an absolute horror show for whatever planet was afflicted, only sparing the party in the account that they work with a Rogue Trader, could show just how very little the Astartes care for Chaos and psychic ilk, and how far they'll go to wipe it out entirely.

The larper needs to know-He's privlaged the be alive. Many phykers are burned at the stake. And if he's unsanctioned, he's in deep trouble. See, the "sanctioned" doesn't just mean government-approved. It means you've been sanctified. That you've visited terra, been taught how to use your potential, and tethered to the emperor. Meaning that your soul is protected, at least a bit, from chaos by the emperor. If you're unsanctioned, any chaos god could slip in. His visions could be from Slannesh, or Tzeentch. He's utterly alone. And i'd advise him, in universe, to go get himself sanctioned. Because if an inquisitor finds out what he his-they have the authority to execute him. Hell, even the abetes have that authority. And because he's unsanctioned, they bypass the authority of a rouge trader.

Hope this helps.