CSA, cannot do that!! I found out from a similar case I was involved in, unless the CSA have being pursuing you and they can prove it they can only ask for a maximum of 6 months to be back-dated.
If they do contact him, he must write to them and ask for a copy of all correspondence they have sent him, they have to provide this info by law. This will be required if it goes to Court, I don't think they will bother to be honest, by the time this has got into the system he will longer be required to pay due to his son bin no longer in full time employment.