I don't know which company you mean by debtline, but they are obviously a fee charging so-called debt management outfit - & if your brother's experience is anything to go by - pretty inefficient.
If he has a written contract with them it ought to include information on how the arrangement is to be cancelled. However, it seems as if they are likely to have breached the contract themselves if they have not been passing on the correct money at the right time, so he should be able to simply cancel his direct debit to them. There certainly seems to be no point going on throwing good money after bad!
He needs some proper free independent advice - could contact CCCS or his local CAB. Some CABs have debt advice information on their websites, including suggested wording for letters and how to prepare a financial statement to send to creditors with the letters.