The original assesment should be redone once CSA receives the correct income information - but I'm not CSA and like everyone else here, I can't tell you what CSA will or won't actually do.
If they redo the assessment, then it will apply from the date you applied for child support because it's within the same financial year. If that happens, then it is likely that he will incur a "child support debt" for the difference between what he has paid and what he should have paid. Depending on the amount of the debt, (which considering the difference in income I'm guessing will be sizable), he could be given an option to pay it off by installments, which would be in addition to the normal child support payments. So I wouldn't be expecting a lump sum.
The best thing to do is wait and see what happens. When the time comes, if CSA doesn't redo the assesment, then there are options available to you to have that rectified. But there's no point worrying about any of this until CSA receives the correct income information.