If he's not earning, he doesn't pay and he tells them beforehand they won't do anything, you just won't get any money so you shouldn't end up in owing them
Push CSA to find out and inform you details off the redundacy package, when his last day of work will be and therefore when you will stop receiving money. Remind them they collect in arrears, i.e. Ex september pay makes up october maintenance.
For example if his last day of work was today, you can expect to receive a payment for next month. If he's then paid 3 month leiu of notice, maintenance will continue for the next 3 months after that.
However getting the CSA to do their job is labour intensive, I have been on the phone to them at least once a week since July. Make it clear he is a shit not a very nice person, as most of these CSA folks seem to automatically assume the NRP is a jolly good honest chap who is being got at be a vindictive witch. Note and get the name of every person you speak to. End the call be reviewing what they are going to do next, ask for a rough timeframe, call to check they've done it at the lower end of the timescale.
Remind them repeatedly that you ex has a legal obligation to tell them what he's doing next, also the company has a legal obligation to answer any queries they may send them (ie. what is ex's redundacy package)
Hopefully your ex won't be quite as clued up as mine as to how to hide who his new employers are (if he has them). My really big battle at the moment is to persuade them not to accept that he's living off partner/family etc. and probably does have a new job.