Colin Farrell "almost broke in half" when his son took his first steps.
The actor's eldest child James, nine, suffers from Angelman Syndrome - a neuro-genetic disorder with symptoms including intellectual and developmental disability, seizures, jerky movements and sleep disturbances - and he and the little boy's mother Kim Bordenave delight in his small milestones.
He said: "We share in the smallest victories; the first words at age six or seven, being able to feed oneself at 19, and getting the seizures under control. When James took his first steps at age four, I nearly broke in half!"
Colin - who also has son Henry, three, with Alicja Bachleda-Curus - went public with James' condition when the little boy was four years old, and
says he was inspired to do so after watching the Paralympic games.
He told the December edition of In Style
magazine: "It was at the Special Olympics in Shanghai in 2007 that I decided, after consulting with James' mother, that I wanted to talk publicly about the pride and joy I had in our son. He has enriched my life, but I don't want to minimize the trials that so many families go through; the fear, consternation, frustration, and pain."
And the 36-year-old actor has been given support in coping with James' problems by the Foundation For Angelman Syndrome Therapeutics (FAST).
He said: "When you're the parent of a child with special needs, it's important to feel that you're not alone."