So sorry about this op
A dominatrix just by the very nature of her job, couldn't be forced in to anything yet alone sex against her will professionally that is.
Your dh has a need to see these women for a reason, his offering to stop seeing them doesn't stop his reason for needing it.
Being an Domme myself in a previous life, I'de venture to say that his relationship with them is very intimate indeed, more so than his having casual sex with a woman. Yes He would have been intimately touched even in some extreme ways depending on what he is in to.
Visiting a Dominatrix can be costly not only monetarily wise, better check any Amazon accounts for wish lists you dont recognise and or purchases. Also check any PayPal accounts.
Does he do a particularly stressful job? Does he manage a large team or is quite senior, often find these types need to be humiliated or dominated. It can their way of destressing and given up control for a while.
Somehow there needs to be a very frank conversation about what he feels his needs are and how she was meeting them, and you may have to sit there and listen to him, if you think you can.
If he doesn't know why it might be that he really doesn't, these reasons are often very buried in the unconscious and go back to childhood, something small like being smacked on the bum once can be a trigger for a life long need.
Either way you don't have to meet his need, you only have to look after yourself and any dc
So sorry lovely 