reduction of arrears

[OliveT]

My partner has a fairly large outstanding amount of arrears to be paid. The amount was calculated for two years based on a certain annual income. He was representing himself (could not afford a lawyer) and she had a lawyer. Her lawyer presented him with an amount for the arrears based on a certain annual income (what he was making at the time of the court proceedings). However he found out recently that he should not have been paying any support while on ei, which he was on for a number of months of those two years after being laid off.
The arrears were supposed to have been settled a year ago based on her buying him out of the house they co-own, however at the time she could not get a mortgage for the house and so my partner did not press the issue as he did not want the children to lose their home. This is kind of a separate issue but applies as it is the reason he was not paying off the arrears. In retrospect he should have just been paying them and worked out the balance with the house later.
My question is: do you think he has a chance of retroactively reducing some of the arrears? He is now making payments towards the arrears anyway but the original amount set out did not reflect his income. Her lawyer did not ask anything about his employment history and he never knew that he did not have to pay support while on ei. My partner’s ex knew he was laid off during that period and so her lawyer should have known this as well and I believe he misrepresented the situation to my partner.
Should her lawyer also have ensured that my partner obtained independent legal advice before agreeing to the amount? Could this also be grounds for challenge? In essence this was a financial transaction.
We are not seeking an unfair adjustment or to wipe out all arrears or to pay less than we should, we are just attempting to make the situation a little more fair.

From what I have read so far, it seems pretty hit and miss whether a reduction is awarded. Any advice would be greatly appreciated.

Thanks!