Why am I paying CSA for my son now he has been laid off?

Had a letter come to my home saying i was in arrears for my CSA £1,800 my 19 year old son started work 03/10/11 got layed off work april 2012 and have been asked to pay the money they have know gone into my wages.

I have been told in a letter and phone call that i have to pay this money as my exwife was claiming child benifit for my son who was working my son asked his mother was she claiming last year she said NO and i have to pay.

Asked can i claim for him from january when he moved in with me NOhe was working?