can I be honest with social worker who is performing a home study interview with me the step parent or is it not confidential?

It sounds to me from your post, that you and your husband need to have a serious discussion about your family and your feelings. If he is pursuing in court trying to get primary placement of his six year old, and you are against the idea of the child living in your home, your feelings need to be conveyed to your husband. He may be spending a consdierable amount of money in attorney's fees in the custody case, let alone his time and effort in trying to secure getting placement, when it appears, you are completely against the idea. Before things get any further along, you two need to talk in my opinion. The guardian ad litem and/or social worker, owe no duty of confidentiality to you, in answer to your question.