Do you have legal representation? Or can you contact the mediator? Honestly, I don't think there's anything you can do until he doesn't pay it, and from what you've said, that will probably happen. Are you going to be prepared to cover it if he doesn't? You can then seek reimbursement, as he will be in contempt of the agreement. Unfortunately, you can't force him to do the right thing, but you can prepare to take care of yourself if he doesn't. If the taxes won't affect you personally, leave it alone. He will have to make his own mistakes. If they do affect you, that is something you may have to take up legally. Unfortunately, MLCers just can't be counted on.