Zigner
Senior Member, Non-Attorney
None of my girlfriends (or even my two fiancées) knew or had access to my social security number. If my case, in a similar situation, they would have to respond as such to the interrogatory.If he has access to it and it is under his care, custody, control, he can be compelled to provide it as the OP is a member of Father's household -- from the sounds of things. It is also possible that it can be held against him since he is going for custody and OP affects that.
ETA: Of course, since there is a child between the two, it's more possible that the OP does have access.