My only suggestion would be to replace actual Daemons (who are all horrendously powerful) with some kind of Warp-echo or lingering seed of Chaos that causes him to hallucinate in combination with his ordinary mental trauma.

Hell, the apparitions could be the ghosts of the people that died on his world and now haunt him with bitter resentment for surviving. There's precedent for that in one of the Ultramarines novels.