A.V.
You either need a current (like within 2 months) form notarized saying he approves of the passport application, and a copy of his ID (front and back) or you need to fill out a form stating why the parent isn't with you and you don't have that information. What my DH ended up doing for SD was meeting his ex at a bank for pickup of the child, and getting the form notarized right then. Locally, there are few places who do passports without an appointment, so you will need to also factor in the time for the processing of the passport and getting the appointment. You may have to seek counsel from a lawyer if he refuses to fill out the form in a timely manner. It took several months before my DH got his ex to go to the notary.
http://travel.state.gov/content/passports/english/passpor...
ETA: While child abductions do unfortunately happen, there are also legit times when a parent is being a jerk and is refusing for other reasons. That would be for the judge to determine. A friend of mine is a ex-pat living in Australia and her DH's ex gave them the run around for a while because she was jealous that they were planning a trip to see my friend's family in the US. There was no kidnapping planned, just a trip for a few weeks over school break. They finally got the passports for the kids when the mother decided she wanted to plan an overseas trip, too. So it was never about them having a passport in general, just not having one the father could use.