Sorry, Just saw this
In a nutshell, NO.
There is already an existing order for wage withholding based on his current income. I have no idea how she will react about your impending nuptual however, so it is always better to err on the "safe" side. Your home and other "real" assets that are co-owned with him will be fine. I don't think you'd need to worry about that.
Now on the financial end, that's another whole different story. It is always best (in my opinion as a second wife) to keep your finances separate when there are child support orders. Why? Because it is MUCH easier to keep them separate in the first place than to try to figure out what's what for the courts whenever it is time for a review.
For instance...The first year my darling second husband and I were married, we filed joint. Big mistake! Because of a temporary lay-off at his job, he got three months in arrears in child support and there was an IRS tax intercept attached to our return. My options were 1. let it slide and allow it pay off his arrears, or 2. claim injured spouse and get my portion of the return back. I chose #1, but the fact remains that if we had kept our finances separate, it wouldn't have been an issue whatsoever, and I wouldn't have been angry at him or his witchy ex for months for taking "my money!"
Now about the other issue of if you have children will his support go down. In a word, NO. Just because he moves on and creates a second family, it does not detract from his obligation to his first child. The only means of receiving a reduction is if his personal income decreases significantly (such as paycut, layoff, job loss, etc) or there is a significant change in circumstances as defined by his court's jurisdiction.
So, on the good side, you are already pretty much aware of what your life will look like with him once you are married. You know what he pays and his obligations. I would also suggest that keep your finances separate, at least until he no longer has a child support obligation. He may be offended, but if you explain why he should understand. I mean I doubt he'd want her to have your money either....
Good luck and congratulations!