Can we get payments out of the mother for when she was the NRP?

My husband has 2 kids and until 2 years ago he was the resident parent for the oldest child for a total of 8 years(5 days per week) while getting access to the younger one 2 days per week – more on holidays etc. My husband paid an agreed monthly amount to his Ex Partner for the younger son but never received any payment for the oldest one in his care. I never understood this – Think he was scared she would try to take him away if he made any demands.

All the while she received all child benefit etc. for both children. A situation arose 2 years ago where she decided it would be better if the oldest boy stayed with her more (4 days per week) thus giving her the right to insist that my husband increase his payments to accommodate the change; which he duly obliged for fear of upsetting her incase she threatened him with the dreaded CSA.

The oldest child is a master in the classic art of manipulating the parents and involving new partners to cause problems to get their own way. The situation now after their latest escpade has hoodwinked the mother who is now threatening my husband with the CSA out of spite and taking all access rights away from him in order to get her own way. My husband has been a loyal caring parent who has been there for his children throughout.

My husband has all the necessary documentation to prove all payments made and is dreading the effect the CSA could have on his livelihood. We are wondering if he is entitled to receive backdated payments from the mother for the period when she was the NRP and made no payments for the oldest child.

It seems crazy that my husband will be getting stung as a result of the mothers horrible actions while she gets away with never having made any payments plus still getting her benefits. Looking for some advice on the possibility of the mother being made to make payments for the 8 years she was the NRP.