If it’s not his child, he shouldn’t have to pay.
She wasn’t pregnant with that child when he married her. What she chose to do after that point, he had no control of. He doesn’t own her.
It was her own irresponsible actions that got her pregnant, let HER deal with it.
If he had married her, knowing that she was pregnant with another man’s child, it could easily be concluded that he was willing to take on the responsibility of fatherhood and being a father to that child. In that case, I’d say, yes, child support would not be unreasonable. If he adopted the child, same thing. Anything else, forget it.
Let him pay. Let her pay.
Divorce filings in my state (Washington) require that the parties declare if the wife is pregnant. I assume that would block this result.