Benefit fraud blocking child maintenance

[Mummaonherown]

Help!
Between July/October last year my ex claimed UC, he was awarded housing costs around 550 and single person allowance, he was laid off and found a new job on 17th October.
I opened a child maintenance case back in August 23 as he wasn't great at paying, I didn't have to use it, or should I say I let him off without paying a few times.
Anyway he went back to work in October (self employed, paid through CIS straight into bank account) and was paying me money every week until December 6th he told me due to Christmas/not working he can't afford to pay for his son and he would resume payments in January (he still got paid up until 23rd December just didn't give me a penny) I didn't argue I didn't see the point.
He kept delaying payments, told me I'd get money last Friday(17th) then said he didn't have it, so I called CSA.
They proceeded to tell me "because he's on benefits he only has to pay me £7.00 a week" I explained he wasn't on benefits anymore (he told me he had come off) but they told me he was lying and he's been paid on 1.11 1.12 and 1.11.
Obviously I was upset, he knows I've struggled especially over Christmas and he's been pocketing 900 extra he shouldn't have, whether he's using that money for his rent I don't know.
They said I had to report him to benefit fraud, so I did on Monday but I was told the investigation could take 6 months.
Child maintenance have said until they get notification from UC all he has to pay is the £7.00 a week, even though he clears £700.00 a week from his job, and worse still if found to be committing fraud by he will have to pay back money fraudulently claimed to the government by CSA will not backdate all the missed payments during this time and will reassess when the notification comes in, so technically my son loses out around £1700.
Surely there's something I can do, this doesn't sound right, my rent is going up in April and my son starts school.
I havent stopped contact I've always encouraged it (he has him 2 nights a month, his choice)
I work part time and life is expensive enough.
How should I take this further and AIBU to expect back dated payments.
I spoke to my ex on Tuesday and he's blamed me for "ruining him" for reporting him.
Anyone got any advice?

Why is reporting somebody for fraud horrible. I view it as abhorrant that a man can't pay for his child.
I'm paying fir his benefits and I don't want to, end of.

I don’t understand this attitude, what if mothers suddenly just decided to pay ‘what they can afford for their child’ even if that’s only £7 a month or whatever? No wonder there are so many shit men about if people find excuses for them. I also don’t understand women who are in relationships with men like this, surely it’s a massive red flag?

Yes, some mothers also aren’t the resident parent and don’t pay, etc. But it is mostly fathers who do this and there’s no point pretending otherwise.

OP, I hope you get somewhere with this.

@NoraLuka absolutely, as a mother I couldn't sit with a man knowing that every penny he spends on me, nights out, my household is money that should be going to his child, wouldn't sit right with me, if he's lying to me he'll be lying to her but I don't think she cares. Good luck to her.

It's a disgrace, it really is.

This is a link to the German child maintenance model: https://handbookgermany.de/en/child-maintenance

How’s it going OP?

You don’t declare earnings to UC, HMRC report earnings to them.

Going through this now sadly, did your MP help at all please?