Farrell, who has starred in "Miami Vice" and "Alexander," said James shows "amazing courage" and is a happy child despite having been diagnosed with the disorder.
"The only time I'm reminded that there is something different about him -- that he has some deviation of what is perceived to be normal -- is when I see him with other 4-year-olds. Then I go, 'Oh yeah,' and it comes back to me," Farrell told the Irish Independent. "But from day one I felt that he's the way he's meant to be."
The actor added, "(I'm) incredibly blessed to have him in my life."
Farrell, 31, shares custody of James with his ex-girlfriend, Kim Bordenave.
He admitted the child's early years have been a struggle, but then noted the family celebrated a recent triumph.
"He took his first steps about six weeks ago, and it was four years in the making," Farrell said. "It was incredibly emotional. There wasn't a dry eye in the house."