Patric acknowledged writing a letter, but said it could never amount to a legally binding "agreement" affecting his paternity rights. "It was a letter ... in a despondent night, a breakup letter, a love letter, this and that, signed by myself. It's not a legal document," he said.
What does it say?
"Oh, everything about our relationship, what we were, my fear of being a father, 'I don't know if I can be a father,' 'I think I can give you this,' 'I don't think we can be married,' all that. But that's nothing. I mean, because after that letter, four months later, we said, 'You know what? Let's do this. Let's go in and sign intended parent with a witness and have this child," Patric said.
His family and friends "absolutely" knew he had a child, Patric said.
Yet his name is not on the birth certificate.
'I didn't make an announcement of it either," Patric said. "I've never had a child. If I can protect him from the bad aspects of my fame that I can, I certainly will. ... This was my idea of trying, and it worked. For two years, there were no pictures of Gus [with] me. None."
Patric reiterated that his intention to stay in his son's life is written clearly -- eight times -- in the IVF documents, where he signs his name as "intended parent" on his donor form rather than waiving his rights. Schreiber signed for the same status.
Asked if he asked at the time what that phrase meant legally, Patric said, "No. What does any ... woman or man think 'intended parent' means?"
"Well, my judge found, obviously, that legally it didn't matter," Patric said.
After losing the court battle, Patric took his battle to the California legislature. But this past week, despite his impassioned plea, the legislation was put on hold by an assembly committee.
It's been 25 weeks since Patric last saw his son, he said.
"The last thing I said to him was, I picked him up, I put him in the car and we drove back after driving to Danielle's, and he said, 'I'm missing you, Dada,'" Patrick said. "I said, That's all right. I'm going to see you tomorrow, little man. And I've never seen him again. "There's not a picture of me in his house," Patric added, beginning to weep. "Forget about my pain, because, you know, I'm an adult and I got in a situation that I should've been smarter about, I guess, or whatever. But he doesn't know. You know, his little furrowed brow wondering where his dad is. ... He doesn't know that. And he's not a possession."
Patric hopes to share custody.
"I want Gus to have the two parents that he had his whole life until he was 2 1/2 years old."
Did Patric wonder if he'd ever see Gus again?
"I'm going to fight 'til I'm dead to see him," he said. "And I'm doing things like this [interview] so he knows I've done everything in my power to make sure he has a Dada."
"It breaks my heart. ... And he has a room in my house, and I don't open the door," he added. "Last week, the maid had cleaned it, and it was left ajar, and I can't go in there. I've forgotten what his voice sounds like."
What would Patric say to Gus?
"I love you. This is not your fault. I promise you I'm going to do everything I can to see you, little man."
Watch the full story on "20/20: Famous Parents, Famous Problems" TONIGHT at 10 p.m. ET.