I am getting married to a guy who was raised by his step-dad. Is there any way, such as through temporary marriage, that my future daughters can be Mahram to their step-grandpa?
Step-dad of your husband in non Mahram to you and to your daughters. If he has a real son who can enter with you in a verbal temporary marriage contract for a short while with out consummating it , that will make his father (the step-dad) Mahram to you and your daughters. If that verbal contract is done with your daughter, his real father ( the step-father) will be Mahram to your daughter.
Wassalam.