For a birth certificate you can order from the dept of vital statistics in the state where child was born. SS I am not sure of but if he had the number could ask local SS office what he needs to get it.
Why would it be needed for DEERS (for a step child) since she has no legal obligation to support her husband's child?
Normally you can get anyone's birth certificate at your local health department. As for the social security card, I would think if your husband has visitation rights to the child that he should be able to obtain that with you local social security offices. He may need to show court papers showing he has legal rights to him.