Facts:
Your assessment is based on your income so no income means no payments due and no arrears build-up. I'd expect you'll have to pay the base liability which is around a fiver a week I think.
The important bit is to tell them now so you make sure your income details are as accurate as possible. Don't simply stop paying.
The other bit:
My statement above is based on the regulations and an assumption from your post that your income will be nil and therefore you can't pay anything. I avoided the moral element deliberately.
Of course, if your income is reducing then an arrangement should be made with your ex. If that's not possible then you need to make an adjustment to your liability assessment with CMG who will communicate with your ex on the issue. CMG will demand you've explored all avenues for reducing expenditure elsewhere; phone contracts, housing costs, loans etc as, quite rightly (procedurally and morally) you ought to be paying everything you can for your son.