even if he has no income at the moment, the fact he hasn't done his tax returns for so long, the CSA are likely deeming him to be trying to hide his income. as a result, they would base it on the higher amount having to be paid. he can definitely ask for a reassessment, but it's up to CSA to deem what it is going to say his assessment is going to be.

FWIW - i have seen assessments go up in arrears (so mum is owed back pay) but generally if a lower assessment is received, it's received from now, not going backwards, so it's unlikely it would be backdated and taken off you