How is a פדיון הבן done with a firstborn son and the father is not Jewish?
To avoid all issues, the accepted Psak is that the local Beis Din redeems him after 30 days without a Bracha, on a tenai, and he redeems himself again without a Bracha when he becomes bar mitzvah.
See references here:
My son was born to a Jewish mother before I converted. After I converted and became Frum, I made a Pidyon for him. I’m told now that I wasn’t supposed to do it. What do we do now? He is already well past Bar Mitzvah.