D.B.
My husband’s ex would not consider any changes in the schedule either, unless it suited her, and even then, if she had plans, she often shipped the kids off to her sister rather than letting them be with their father and me. I know a big issue was her fearing they would want to live with us, and that would hurt her ego as well as her child support income. So we learned that there’s not much you can do if there is opposition; at the time (maybe not now), the courts also saw that people who couldn’t agree should have the joint custody reversed since “it wasn’t working.” So that would have been worse. I also know that I had to completely stay out of things – any suggestion that remotely sounded like it came from me just set her off. I don’t know if any of this is at play in your situation.
I agree with JB that you aren’t really getting more time –just fewer transitions. Not that it’s a bad thing to minimize the disruption, mind you. Also, it may be that he is saying he wants more time with his father (because he truly feels that all the time, or at the moment…or because he feels he’s supposed to say that) and therefore he may be saying the same thing to her (either because he feels it, or because he thinks she needs to hear it). Kids try to keep all their parents happy. She may be refusing you because she’s insecure or a big pain, or because she is standing up for him (or thinks she is).
What you might start with is some subtle adjustments to the existing schedule – keep him longer once, but return him earlier the next time. Over time, it may show her that you aren’t a threat to her or her time with her son, but just willing to be flexible based on events, whether he’s sick and under the weather, or whether her family has visitors (or yours). Yes, that means facing rejection from her, but if her ego is fragile here, you might build it up over time. Tween and teen boys often are more drawn to their fathers, and he may want to come more (and stay more) the older he gets. That may be threatening to her. At some point, the court will listen to the kid, but if he’s torn now, it won’t go your way and it will just cost money and create animosity.
Good luck – I know it’s hard.