CSA making partner pay because stepson doesn’t want to work

Stepson left school left his mums and lived with his dad for 18 months and went to college went back to his mums for a year did scrap metal worked on fairgrounds then at 18 decided to go back to college as couldnt be bothered to work and csa has billed his dad 5k for a years course , to me this is disgraceful he has to pay his ex wife £400 a month because his son cant be bothered to work.