The TARDIS' panic seeps into him without any trouble at all, and as the surroundings change, he backs away further. He's sure that TARDIS could hurt him here, if she wanted to, and the less rational she becomes, the more likely it is that she'll lash out with more than illusions.

He stumbles back in the darkness as the enormous figures advance on him. He doesn't change his form to better fight them. He needs to get across to the TARDIS and this is the best way to do that. "TARDIS, stop! The rift is going to kill both of us if you don't let go!"