Why am I paying for another father?

Iam am paying csa to a 18 year old father of one .i have phoned and told csa that he is a single father and looking after his child.

He is not in any education as that finished in june 2012 he is now 18 and must be claiming somthing for him and his son .he does however still live with his mum and they seem to be getting around this!

I have told csa about this problem and all they tell me is as long as his mum is getting child bennifits for him i still have to pay csa how can this be he is a adult with a child???????