Ok. You're conflating two issues.
CMS is calculated on your earned income. There is a flat rate if you are on benefits, but only if you have no earned income either.
If his entire income is £1200 benefits a month (what is this by the way because that's more than full level daily living component and mobility component of pip combined) then he will likely pay the £7.
If he still gets the pip, which as pp says, you can be a millionaire and claim it, but he earns £30k a year, he is liable to pay CMS on that full £30k (they might give a small allowance for pension deductions).
If he has rental income, or income from other assets then you can contest the calculation. As for a variation. Raise a high level complaint (in CMS this is called a dissatisfaction) and do not confirm you are satisfied with their dealings until the FIU is involved and pull him in for interview. Raise dissatisfactions against the FIU if they are dragging their feet. They start becoming very compliant with what you need to get that dissatisfaction off their case record.
So your issue is not the benefits. Your issue is the undeclared income that should all be liable to CMS. The difficulty is now proving it because it's cash in hand. Keep reporting to HMRC. On and on and on. Take photos of him on the roof. Report to pip fraud. Report it to your MP. They have a direct line to CMS and can put a fire up them.
I've just had over £40k arrears from my twat of an ex because they told him they would sell his house and sent it to court. After 6yrs of CMS doing utter fuck all, I found out about the MP angle and the dissatisfaction thing, and he was hauled in under threat of arrest to the FIU and my money was with me within a year.