My husband and I are talking about leaving a double portion for the firstborn son. He was previously married and had a daughter, who passed away at 3 years old,then a son who was born via c section.In our marriage, we have a son together, born naturally, then, another son, plus 4 daughters, all 6 born naturally. Who would be the firstborn son in this situation? Thank you.
In your situation there is no Firstborn who had rights to a double portion by Torah law.
Torah law awards double only when the firstborn child of the father is a boy. In this case it was a girl, this removes firstborn status from all subsequent boys, even after her death.
See Shulchan Aruch, CHoshen Mishpat 277