Right then, well, he'll just amuse himself with his non-pattern-recognizing toy as the impulse moves him. And trust to his own timing instincts to end the flow shortly before he would otherwise start to feel tired or burned about it.
When he's done he'll let her out cuddle up to her and close his eyes while she's still in chains, whispering that she doesn't need to say anything. If he let her out, or made her talk, she'd probably look safe or protected or happy and Keltham can't make himself do that, not on purpose, not by his own acts, even if it needs to happen eventually. At some point she'll need to go to the bathroom, and then she'll have to talk, or she'll have feelings to talk about, or something. Until then Keltham can't make her speak.