I seen a lawyer for this. Don't put his name on anything of yours. Bank accounts, deeds, stocks,bonds. These things are called attachments which means they are attached to him through you. They can take money from these things if his name is on them too. Your tax returns also file injured. But keep in mind if you do file joint, if you can prove what part of the refund is yours you will get it. I have been through the whole thing. In all these years none of my income was touched.
AS long as you don't sign anything with his name on it don't even file joint income tax they will take anything with his name and your name attached, so file separately have single bank account without his name on it......the best to you if you wish to help him pay it use a money order......
You are not responsible for his back child support . But don't file any taxes together or they will take it since it has his name on it . This happened to my brother .
As soon as you are married then you will both be responsible for the payments I'm afraid so they can take it out of both of your money. Sorry.
If you do file joint you can file an injured spouse form with the irs ...stating that the debt that was taken from your return was your spouses...