We'll exclude cases where they know it isn't theirs but choose to raise it anyway (adoption, step kids, etc). I'm assuming they know if they adopt a kid it isn't their kid biologically.

But in cases where they very much believe this child is biologically theirs and it isn't should they be told?

Let's also limit this to cases that are discovered after the child is born and is still fairly young.

Do you tell that "parent" because they should have the truth or keep it a secret for the good of the child/family?

/And yes this will necessarily be a primarily male concern due to biology. But it could also apply to women in cases of hospital or fertility clinic error.