Your step-sons Birth certificate & his Mothers Death Cert.should be enough to establish his late mother as the biological mother, Her death certificate should be enough to establish that previous marriage has ended.
Keep in mind you will need to complete the I-130 for your husband and a seperate one for your step-son
I am not an attorney however logically the death certificate of your husbands late wife and your sons birth certificate should be enough, there should be no reason for DNA test. I would assume Your step-son is considered immediate family if you are married to his father
I suggest you call an attorney and explain the situation, this case is unique and you may need at minimum an attorneys initial guidance.
Good luck & God Bless