would i have to add my fiance's son to my health insurance plan?

my fiance pays child support for his son from a previous relationship. his job doesn't offer health insurance. so if i add him to mine, as a "domestic partner", would i also have to add his son?
--------------------
Since the child is not your dependent, you cannot include him in your insurance. You are able to include the father because you are mutually supportive and dependent on each other, but that does not extend to his children. If he is required, by the courts, to provide health coverage for his son, it is not your responsibility, it is his.
Source