Child support agency arrears - deductions order

Janjel

My brother received his wages today with a £125 child support agency deduction. He knows nothing about this as his son is 18 years old and he had an agreement with childs mother, also no letters or phone calls. He rang child support agency who explained this was arrears from years back and that they had sent hime letters along with a letter to his wages department at work, they also did a follow up call to them. They mentioned nothing to my brother, just deducted from salary. He has no dispute over money as if he owes it then he is ok paying it but thinks his work wages department should have said something before deducting anything from wages.

Any advice would be great