03/19/2014 at 05:00 PM EDT
In his Santa Monica, Calif., house, Jason Patric takes a seat in a bright, cheerful child's bedroom stocked with stuffed animals, books, baseball gloves and toys of every description.
But he hasn't seen the boy the bedroom is meant for in more than a year.
"It's been 54 weeks since I've seen Gus," the actor says an emotional interview in the new issue of PEOPLE. "The hardest part is thinking what he's feeling. Every single day there's a new wonder and worry: 'Is he ok? Is he hurt? What does he look like? What is his voice like? How is he doing in school?'"
Barred from seeing his biological son, now 4, the star best known for 1987's The Lost Boys and 1997's Speed 2: Cruise Control is locked in an ongoing legal battle with his ex-girlfriend, Danielle Schreiber.
She says Patric, 47, simply served as a sperm donor when Gus was conceived through in vitro fertilization in 2009. Patric says he always intended to be part of the boy's life and formed a deep bond with him in the time they spent together from the day he was born.
In February 2013, a judge ruled that Patric had no parental rights according to California law because Patric and Schreiber were not married when Gus was conceived. (Patric says he and Schreiber signed forms listing them both as an "intended parent," but the judge found there was no valid coparenting agreement in place.)
He has since filed an appeal.
"My love for Gus is what keeps me going," Patric says. (He adds that he financially supported Gus, including paying for the child's private school, but Schreiber disputes that, saying he was repaying money he owed her.)
In PEOPLE's new issue and an accompanying video interview, he talks about his case and the devastation at being separated from his son.
"I have so many sweet memories of Gus," he says. "Just walking on the beach, sitting on my lap, looking at airplanes ... teaching him who Elvis was, rocking him to sleep as we played 'Somewhere Over the Rainbow' ... Every single bit of wonder that you could see in his eyes every day."