My ex has put a claim in for my son who I haven’t seen for four years

I am a single mother of two children,my daughter 14 lives with me,My Son 18 lives with his dad&new wife and 3 children. My ex is unemployed, my 18 year old son at college and works partime.

I only work 2 days a week,but have never claimed for csa and my ex has not seen my daughter for 4 years.But my ex has now put a claim in with csa for me to pay for my son whi ive not seen for 4 years(father would make sons life hell if he was to see me).I can barely pay for the roof over myself and daughters head as it is but now CSA want to take 20% of my wagesI will not be able to keep my home.I have no family in the area to help.My ex is unemployed so I cannot counter claim for my daughter,i truly am scared stiff as to how I am going to be able to care for my daughter.My ex is currently in Turkey on holiday while I am here worrying!