Not to put too fine a point on this, but I think if cuckolding were the case, the element of surprise (at the paternity results) would not have been part of the equation. (assuming there was surprise, I did not bother to read the whole thing, in temporary violation of one of my own pet peeves...)
Did you mean to say "WOULD have been part of the equation"?
If the child WAS sure to be his, the mom could have instantly relieved his suspicions by simply saying "Of course it's yours, and to prove it we will have a DNA test done tomorrow morning".
If she had instead said "Too late, sucka! You have acknowledged paternity, so you are stuck with child support for the next 18 years, and there's NOTHING you can do about it!", then his blowing her away is understandable.