File a Motion to Modify ASAP. There is an order right now that says you pay child support. Technically, even if the child comes live with you, that Order is still in full force and effect. Many people swap custody and just deal with it themselves with no problems but when this happens and it blows up (and it does) I assure you, the cost of fixing this is very high. DO NOT SIMPLY IGNORE THE ELEPHANT IN THE ROOM THINKING A JUDGE WILL FIX IT LATER! I cannot tell you the number of time I have had fathers come into my office with the opinion that the Judge will just fix something that happened years ago. A Court can retroactively fix something only if: equity demands it AND the party seeking relief has made reasonable efforts to get relief. The legal theory is called "latches". Latches (and this is over simplifying it) can be thought of as leaving a door locked even though you have a key and then complaining later that you were locked out of (or in) the room. You had a key, why did you not use it? Why should the court help you if you had the key and did nothing? Legally speaking the latches defense is much more complicated but I hope this analogy gives you the point. Do not sit on your hands and assume the Court will figure it out later. Do something. You have the child, file a Motion to Modify, get Mom to agree if possible and if not, have the Judge explain to her what is going to happen. She is owed back child support, you still owe back child support, she should be paying you child support now. The Court can and likely will simply offset. Rather than you paying her $300 a month towards arrears and her paying you $300 per month for support, you get nothing, she gets nothing and the books balance out over time.
Answer Applies to: Texas
You need to file a motion with the court to modify the child support order, stating that your son now lives with you, and asking the Court to order your wife to pay child support to you. Alabama courts will usually order child support even if the non-custodial parent is not working, because the courts assume that a healthy individual can find a minimum-wage job. If the Court orders your ex-wife to pay child support, you may be able to accept a credit against your back support in lieu of actually getting money from her.
Answer Applies to: Alabama
You will need to file a motion to modify the current court orders. The support order remains in effect until modified. You cannot retroactively modify amounts owed prior to the date you file a modification action.
Answer Applies to: Texas