You will need to go to court and have a formal stipulation signed, please work with a local attny, take care.
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Having proof he will and actually doing it are two different things. A name change is much easier than an adoption as far as whether you need the father's consent, at least in Ohio. See a local attorney.
Although your marriage is not (yet) recognized in PA, but adoption is possible - PA does allow adoption by same sex couples.
This is not intended to provide legal advice about your situation - just a few casual remarks about a legal question. We are a debt relief agency helping people file for bankruptcy.