Financial abuse? Am I overreacting? How do I navigate this calmly

[Karou]

TL DR
Am I being unfair to refuse to show X receipts when asking him to cover costs relating to his son?
How do I address the subject of child support with him? Do I have to justify everything I spend money on relating to our son and show receipts?

A little history might help as I may be being over sensitive.
Divorced X 4 years ago, throughout 14 year marriage he was controlling about money, we had a joint credit card, not bank account, but if I spent any money on it he would demand to know why we needed this, that and the other. I only ever used it for grocery shopping but got so fed up with justifying my spending that I just left that job to him. The house was in my name for various reasons so he didn’t pay the mortgage or any of the bills and, after he took over all the grocery shopping, refused to contribute to the bills.
In the divorce I got half the house, he took everything in the house that he had paid for, apparently that made it his. For 3.5 years we shared custody of our son 50/50 so no deal on child support.
X is now retired but has a pension.
6 months ago DS, age 14, has decided that he wants to live with me, see his dad every other weekend. His dad has made no offer of child support. He has paid half of things like school trips. He has said that he will contribute to costs if I ask him. Last night I sent a text to him asking him to transfer £120 to me to cover extra expenses related to DS going back to school (uniform, train ticket etc). He texted back demanding to see the receipts. I refused, mainly because I have been accumulating this stuff over the past few weeks as part of grocery shopping and don’t want him going through my receipts, but also because I have a gut reaction that his request is utterly unreasonable. He honestly seems to think that he has made a perfectly reasonable request. I listed off what each item cost and he has said that he will transfer the money this once but he expects receipts in the future. I did lose my temper and accuse him of financial abuse at which he tried to turn the table and say it’s me being abusive. I shouldn’t have done the last bit, my bad.

Op are you concerned you will get less from cms? Because whay you’re doing is completely illogical and if I was him, I’d find it bloody annoying. Is your aim to cause contention? To get his attention? As you’re not behaving rationally.

go to cms. You get a set month each month, then manage your budget accordingly,

Either cover up the receipts so he only sees the relevant bits, or buy stuff that will annoy him either side (will take some careful arranging at the supermarket checkout).
But an itemized list ought be fine, I can see his point re exact pricing, but demanding proof is a bit OTT.

The CMS take pension income into account if it's over £100/week. That would be by far the easier route to go down and not subject yourself to his ongoing abuse. They will investigate his income and set the amount (I think it's 16% of gross income before tax). If he refuses to pay voluntarily they can garnish his income and charge him an additional 20% for the effort. I would not hesitate to do this in your circumstance. Continuing to endure the behaviour that contributed to the marriage ending is not good for your mental health.