Can't I use that against her, so she will let me see my son
Look up blackmail. What you are suggesting could be a crime.
However, what are the circumstances of her not filing taxes? Did you file a return as married, separate and she didn't file for her income or did you expect her to file a return for your income?
If you filed as married, filing separately and she simply didn't file for herself (and if she was obligated to do so on the basis of her income), then you should ask your attorney whether you need to report her in order to protect yourself. Of course, if neither of you filed taxes, you need to get that taken care of. It's pretty hard to drive a truck from jail.
Let us know exactly what's going on with the taxes.
Taxes have nothing to do with it.
What matters is the length of the marriage, your respective incomes, things like that.
Specifically:
Alimony in South Carolina