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from what I understand......
As long as 10/11 FY was based on his 09/10 tax return, there won't be a problem.
As you said, child support is based on the previous FY tax return, so his increased income will start to effect the payments you receive from September 2011.
If his income had happened to decrease, by 15%, then he would have been able to ask for a reassessment and provide an estimate so that his payments were lower.
If he had provided an estimate of his income for the year, and it was lower then what his actual income was for that year, then he would have a debt - however I am unsure how, or if, his tax return would be used to pay that debt if you are on a private collect arrangement.