Well it sounds as if he has a business anyway. Normally capital gains tax is payable on the profits from buying and selling property (with one's own home being exempt). And if it's a regular business, then he will havae to pay income tax as well (or instead of - I'm not a tax expert). So the HMRC will either have a tax record for him, or if not, you might want to alert them to that possibilty that he is evading tax. The reason I mention HMRC is that the child support system changed recently, and the assessment is now based on someone's gross earnings as recorded by HMRC in the last tax year, rather than under the old system , which was based on net earnings, and relied on the person themselves saying what their income was.
Of course, if he's paid cash and doesn't declare it, then the HMRC won't have a record - but then they wouldn't have had under the old system either.
The problem you've got with the new system (which is called the Child Maintenance System (CMS) rather than the Child Support Agency (CSA)), the emphasis is on separated couples reaching agreement between themselves. You can still go to the CMS to get them to assess your ex - but I think there is now a charge to pay if you want to take that route.
Here's a government website that gives a lot of info:
I hope this helps and I wish you the best of luck.
Thanks and best wishes...