OPP-How Can You Co-Parent Christmas?

A woman had an agreement with her ex husband about Christmas presents for their son. He decided not to honor that. Is there a way to co-parent Christmas?

I need some advice. My ex husband and I have been divorced for 5 years now. We have a 12 year old son that we share custody of. For the most part we have always got along. We're friendly to each other. I would say we're very good at co-parenting. When it comes to Christmas we have always agreed on what the big gift should be that our son gets and we split it. One of us will go buy it and then the other pays that person for half.
Back in October my ex and I decided this year's gift would be a smartphone for our son. We both looked into our own phone plans and decided that I should buy the phone on my plan and then we would split everything 50/50. He would just add in the monthly additional cost to his child support. I went and picked up the phone last Monday. My cell phone carrier was having a Pre Black Friday deal. We then we're going to give it to him at our exchange near Christmas.
My ex had my son for Thanksgiving this year. When my son came home on Sunday night he was so excited to show me his brand new iPhone that my ex and his new girlfriend bought him on Black Friday. I was livid. I tried not to let my son see how upset I was but it was difficult to hide the shock.
I immediately called my ex and asked him what was going on. He played dumb and claimed he didn't think we had any set plans this year for our son's gift. He said he and his new girlfriend are allowed to buy him anything they want. He doesn't owe me any explanation about what he does. When I told him I already purchased the phone he said, "Not my problem."  Plus he said his girlfriend's cellphone plan offered an amazing deal and so our son's phone is under her plan. That doesn't seem right to me either.
I was so hurt and so shocked. My ex has never acted like this to me. We have always gotten along so well. He looks like the hero with our son and I'm left with a phone I can't return and a bill I have to cover at 100%. That leaves me with no money to get my son something nice for Christmas. Is there anything I can do? Should I insist my son's phone plan be moved to my ex's or mine and not his girlfriend's? They've only been dating for 6 months. How do you handle being one upped by your ex at Christmas?

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