Have you considered negotiating a temporary separation to give him a chance to be with only her for a bit, with a time limit and conditions?
Is the relationship really over if he chooses her, is it a deal breaker? Say if he decided to come back to you, would you take him?
Is he a burden to your happiness in any sense, financial, emotional, spiritual etc.?
Is this relationship two way, does your happiness matter at all or is he trying to control the reactions of the two people he wants so he doesn't have to feel conflicted or uncomfortable?
If he leaves, do you plan to withdraw your financial support? If so does he know this?