If we restrict ourselves to legal options, he can hire a lawyer, which is what he should have done rather than "ordering a DNA test". As a general rule, the minute you get served with a lawsuit, you should hire an attorney, especially if you are a billionaire. He should not volunteer to get in this position by volunteering for the test. But given that he did let it get this far, and now it is a legal fact that he is the father of the child, his newly-hired attorney would try to put the best face on this mess that he can. It's not a foregone conclusion that a child would be entitled to $20M. Also, is that per month?
The amount that the court would order depends on the particular state's law. I assume this is in Washington state since you didn't say otherwise (subsequent change of venue notwithstanding). The calculation is based on a maximum joint income of $12,000 per month. The mother cannot just quit her day job: the courts can impute income to her. I assume that she makes $3,000 per month, which is a low-ball figure. His obligation would be about $1180 / month, which works out to be about a quarter million total over the life of the support order. The courts do not have the authority to award $20M, nor do they have the authority to order a lump sum. They can award $1180, and this can be increased or decreased depending on circumstances.
The attorney would also argue that he is legally not the father, because under the law, this is "assisted reproduction" since it does not the result of sexual intercourse, and mere sperm donors are not legally deemed top be parents.
Incidentally, the benefit of "marrying rich" are distinct from the benefits of a rich baby-daddy. Child support is for the support of the child, not the benefit of the mother. The attorney could therefore persuade Mary to accept a lesser amount, no questions asked, to dispose of the problem. The net financial benefit of this dumbass scheme is probably substantially negative for her. A third option is to hire detectives to prove that this was a fraudulent action, which would lead to her imprisonment.