Farrell, 48, told People magazine in an exclusive interview that when James was first born, he seemed like a "chill baby" who didn't coo or make any vocal noises at all.
By the time James was 18 months old and hadn't hit developmental benchmarks like crawling, being able to stand, or staying in a sitting position on his own, Colin thought something was off.
James was initially diagnosed as having cerebral palsy, a common misdiagnosis given the similar symptoms shared with Angelman syndrome.
A year later, he was properly diagnosed with Angelman syndrome by a pediatric neurologist who spent more time with James and noticed he exhibited a characteristic of the disorder medically known as "inappropriate outbreaks of laughter."
Frequent seizures are also common with Angelman syndrome. Farrell said that James suffered many seizures at a younger age that resulted in ambulance rides, but hadn't suffered any since his last episode ten or eleven years ago.
Farrell, who fathered James with American model Kim Bordenave in 2003, also shares a second son, 14-year-old Henry, with his ex Alicja Bachleda-Curuś, a Polish actor.
The In Bruges actor decided to share his family story in the hopes of raising awareness through his new Colin Farrell Foundation, an organization dedicated to James that helps provide support to families and children with intellectual disabilities.
"Once your child turns 21, they're kind of on their own," he told the media outlet, referring to the cut-off age in the US when special assistance programs and services are no longer offered to families and children with additional needs.
He explained:
"All the safeguards that are put in place, special ed classes, that all goes away, so you're left with a young adult who should be an integrated part of our modern society and more often than not is left behind."
Farrell was inspired to launch his foundation after having the unsettling realization that he and Bordenave would not be around forever to take care of James as his general life expectancy is not compromised by his condition.
"It's a terrifying thought that I'd miss the last 40 years or 50 years of his life because I won't be there to shepherd and to protect," Colin said, adding that his hope for James would be "a life of connection" and that once his parents are gone, James will "be somewhere he feels like he belongs, where he feels safe."
One particular moment that got Farrell emotional was when he recalled witnessing James' first steps two weeks before his fourth birthday.
It was a memory Farrell said was an "overwhelmingly beautiful" moment. The actor and his family had been previously warned that those with Angelman's syndrome are unlikely to walk.
"I'll never forget just the face of determination on him as he walked toward me. He took, like, six steps, and I burst into tears," he said, adding:
"I'm proud of him every day because I just think he's magic."
Farrell expressed gratitude for having a successful career affording him a live-in caregiver, who has taken care of James for 18 years so Farrell can "move freely and work" when father and son aren't engaging in regular activities like watching movies, playing ball, swimming, and grabbing In-N-Out Burgers together.
"It's been great for James as it gives him that autonomy he knows I'm here. He goes to the beach, he goes to Universal Studios. He has a good life. He's a happy young man," observed Farrell.
He noted how James as an adult works harder than anyone he knows, including himself, because his son is very determined.
"It'll take James 90 seconds to take a T-shirt off," he said. "But he takes a T-shirt off; you sit it out with him."
"If anything, he also gifts me with the ability to look at the human being and the human body and life as a marvel, because I see how much he struggles with things that I'd never have given a second thought to, that many of us take for granted."
However, when it comes to the future, Farrell has a deep concern about James being left on his own.
"My fear would be, God forbid, if and when something happens to James's mother or me, James would be 30 or 40, and then he'd have to go in somewhere," Colin said.
"Into some kind of institute or some kind of residential care at 30 or 40, and there'd be nobody there to call over and take him out and have lunch and all that."
When asked what he is most grateful for, Farrell, who was known for his party boy past, borrowed from fellow actor Jamie Lee Curtis' answer of sobriety.
"James was about 2 when I got clean, when I got sober, and he was a big, big part of me putting the bottle down, a big part of it," he said.
"Because I was in no condition to be a friend, never mind the father of a child with such exacting needs."
"If it wasn't for my sobriety, I wouldn't be able to be there for James and enjoy in the marvels of his life and support him in the way that I feel that I can."
Farrell said that for years he had endeavored "to do something in the realm of providing greater opportunities for families who have a child with special needs, to receive the support that they deserve, basically, the assistance in all areas of life, whether it's education, intervention, therapeutics, or social environments that their child can feel like they belong within" so that families and children with special needs can "find a world of meaning and a life of connection for their children."
According to the Colin Farrell Foundation's website, the organization "is committed to transforming the lives of individuals and families living with intellectual disability through advocacy, education, awareness, and innovative programs."
"People living with intellectual disability are facing multiple crises that have deleterious effects on their health and well-being."
You can help the Colin Farrell Foundation's mission to raise awareness "of the challenges and crises people living with intellectual disability face each and every day" by making a contribution here.