Can a divorced man win a downward modification of alimony and/or child
support if he were tricked into a marriage by a child that wasn't his?
The couple then gets divorced and the wife sues for alimony and child
support. Will the man likely be able to get reduced child support
and/or alimony on the claim that he was tricked into a marriage he
would not otherwise have undertaken?
Generally speaking, marriages induced by fraud about anything other than the extreme case of fraud involving who someone is marrying in the act of marriage itself (i.e. someone believed they were marrying John Smith from Denver but actually was married to John Smith from Los Angeles, whom they had never met before, because the marriage ceremony was conducted with disguises and they didn't look closely at the marriage certificate), has no impact on marital rights. Evidence regarding fraud to induce a marriage wouldn't even be admissible at trial because it wouldn't be legally relevant.
This has no impact on alimony.
Paternity can be contested by filing a paternity suit within the statute of limitations for doing so which varies from jurisdiction to jurisdiction (usually within two to five years of the child's birth). If the husband prevails in a timely paternity contest, child support will not be owed. Otherwise, actual genetic paternity would be irrelevant.
In California, the statute of limitations to disavow paternity of a child born to one's wife is two years from the date of birth.
N.B. I am making some interpretations of inexact language in the question. It says:
The couple then gets divorced and the wife sues for alimony and child
support.
But I assume that what is really meant is that one or both of the members of the couple file for divorce and that in the course of the divorce proceeding the wife seeks alimony and child support. If the divorce proceeding is concluded without an adjudication of paternity that would usually preclude a later lawsuit to disavow paternity.