Nonsense. They can do paternity DNA almost right away.
And vasectomies are reversible, to my knowledge.
If you’re talking about doing a DNA test on the fetus, that would require permission from the mother, who in this case, would not want to give it. She knows the honest man is not the father; she wants to blackmail him. But the court can only require a DNA test when the baby is born.
Vasectomies are *typically* done this way, which makes them ‘more’ reversible, no guarantee. All vasectomies occlude (i.e. “seal”) at least one side of each vas deferens.
http://i.imgur.com/xVf4iVv.png
However, if the surgeon doesn’t just cut the tubes, but removes a section from each of them, it will be much harder to rejoin them after removing the occlusions. Doing so helps to insure that the sealed tubes do not go septic.