Colin Farrell cried when his son took his first steps.

The 48-year-old actor's eldest child James, now 20, suffers from Angelman syndrome - a rare genetic disorder which involves delayed development - and he will "never forget" when the boy walked unaided for the first time, two weeks before his fourth birthday.

Colin recalled to People magazine: “[His occupational therapist said] 'James has something to show you.'

"I knew they were working on walking. And I stood over there, and she let him go, and he just came to... I mean, it was so profound. It was magic.

"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."

The 'Lobster' star told how a lot of families with children who have Angelman syndrome are told their kids may never walk.

He added: "So to see those first steps was just . . . I’ll never forget it. It was so overwhelmingly beautiful."

Colin praised James - who is non-verbal - for having "worked so hard all his life, so hard" and he feels proud of his son's achievements every day.

He said: "Repetition, repetition, balance, his jerky gait.

"When he started feeding himself for the first time, his face looks like a Jackson Pollock by the end of it. But he gets it in, he feeds himself beautifully.

"I’m proud of him every day because I just think he’s magic."

The 'Banshees of Inisherin' actor - who has James with Kim Bordenave and 14-year-old Henry with actress Alicja Bachleda-Curus - told how his son's condition was initially misdiagnosed after he failed to hit developmental milestones.

He said: "He couldn’t sit up. He wasn’t crawling. I think he was a year and a half when we took him to get really checked out, and he was diagnosed as having cerebral palsy.

"It was a common misdiagnosis because it shared a lot of the same characteristics."

The Irish star is now launching the Colin Farrell Foundation - which will help young adults with intellectual disabilities through advocacy, education and innovative schemes - and he knows it is what his son would want him to do.

He said: "I have to make a call based on knowing James’ spirit and what kind of young man he is and the goodness that he has in his heart.

"I choose to believe that if he knew us doing this could help families and other young adults who live with special needs, he would say, ‘Dad, what are you talking about? Why are you even asking me? It’s a no-brainer."