I’m a benefit fraud investigator. The IR and CSA could find him and work out his income but they don’t have the resources to do it on everyone.
The basic things investigators do are credit searches, you just need his name, d.o.b, last known address. That should show his current address and any credit he has applied for.
We use legislation to allow us to look at people’s credit history but pi’s just log onto experian and pretend to be the person they are looking to trace.
If he’s just bought a fancy car on credit I would then look to see what income he declared to get his finance. Obtain his bank statements and see what income he has go through his account in a year. Look at his expenditure etc. pi’s can’t use legislation but they have other means.
You could send any information you get on him to the CSA. If he has made any false statements about his income they would look to prosecute him.
Good luck