That is good news.
You need to apply on a D11 and ask for Costs to be assessed if not not agreed
You will need to attend - and take with you the Solicitor's bill which must be limited to the costs of the divorce alone - not any financial or other negotiations
The court knows what the Court fees are!
Please ask if you need further details
The Court will take a written representation for your ex - saying that he cannot afford it will not be a proper defence
If they are the solicitors he is using for the divorce and they are on record at the court then that is fine
It would probably be pointless the Judge will not know unless it is already on file - which is unlikely
However you can call into the Court Office and ask.
You can enforce it whenever you wish - yo can attatch his earnings if you wish - or you can wait until the final order has been made