Ex lied to get son to move in with him and now wants CSA from me!

I have bought my 3 children up alone for the past 9 years and their father has avoided any payments by working cash in hand,he recently told my son a wicked lie,he told him that as a baby I tried to have him adopted,my son has now gone to stay with his dad and rarely talks to me even though I have assured him it is all lies,now my ex has applied for my boys tax credits, child benefit and also maintenance from me!!!!

Csa cannot or will not do anything even though he has 2 new cars on finance he claims he lives on less than £7 a week. My heart is breaking and I’m at my wits end. Please help someone or I don’t know what i will do.