He will have to have proof of any voluntary payments he has made. The CSA does not just take his word for it! The proof he needs is your agreement that these payments have been made or bank statements showing payments to yourself and that you accept these payments as child maintenance and not spousal maintenance or pocket money for the kids. His arrears will continue to build until he starts paying his regular maintenance (the weekly assessment). The agency's debt enforcement section will pursue the arrears and there are several ways this can be done. If he is employed an order will be made to his employer to deduct 40% of his take home pay. They will also go to court for a Liabilty Order. If he lives in England or Wales bailiffs will be instructed to visit him with a view of seizing goods to the value of the Liability Order, In Scotland Sheriff Officers will be instructed to do the same although they can only seize goods from outside his home like his car. If he owns his own home in England or Wales the CSA can force sale of his home; in Scotland a charge will be made against his property. If the property is sold the CSA will have first claim.
Hope this helps.