He's always stone cold, and you can just write that off as stoicism.
But in this case, it's explicitly about his confidence in resisting pleasure, so it especially comes off as him just not really enjoying sex.
I mean - for him, sex is a means to an end. Literally, how he gains power.
He's the anomaly of that world, being a guy who willingly puts out for women, and it's more like training sessions for him than anything explicitly pleasure-focused.
Plus, as far as the meta-call of "smut being weird when the guy looks stone cold", I would put forward that, for a title like this specifically, it's not the guy you're meant to be focusing on during the sex scenes.
So whether he looks caught up in the throes or not, is immaterial to what I imagine the author expects the reader to focus on.