It sounds like your DS is about 16? He's old enough for you to sit down with him and explain the financial facts of life.
If he needs something, you & X will do your best to get it for him; it may not be the most elegant choice, but it will be adequate.
If he wants something, he has to get the agreement of BOTH parents without manipulation. Even if STBXWW doesn't play nice, he has to understand this dynamic.
Yes, this falls into the category of 'co-parenting with a lower muppet', but you have to make the best of it with your kids.