Concept: the biological "Flynn" father is an Irishman coming from a small-town family - the elders of whom stay very superstitious. Father Flynn grew up being fed bytes and proverbs about fairies and tricksters and banshees and the like. His grandparents even told him "true accounts" from their own youth that were filtered through years of foggy memory.
But he, himself was a modern man, and moved to the states with plans to stay there forever as a hardworking officeworker. He was living the dream until his second child was born. And slowly, twenty years of stories come back to him. His grandparents always foreboded the danger of changelings, and he never actively believed them.
But here, here's his own son... And strange, unexplainable things are happening. His wife Linda doesn't believe him when he swears that Phineas does these things. Odd things that no infant, 1 year old, 2 year old should be able to do.
Linda didn't believe him when he sat her baby boy down by the piano to occupy him, only for Phineas -having never been to a single lesson in his paltry time on earth- to make up a plucky little song on the spot. Complete with tiny, whimsical vocals.
Linda didn't believe him when he turned his back on his notes for only a second, only to return and find a tiny Phineas reading, no, pouring over his writing with the discernment and intensity of an expert in Flynn's own field.
She also didn't believe that he - with the help of their NORMAL daughter Candace - himself didn't write Phineas's twenty-page short story detailing their family going on a magical adventure through the Danville park, in shaky crayon letters.
He'd had a lot of respect for the elders of his family, but he wasn't a superstitious man before his son was born. Afterward, though, he became almost paranoid... Watching over his shoulder as the boy did things no human child should be able to do - whilst the child watched the Flynn Father back with an absent smile, and knowing eyes that told the man in no uncertain terms that he was FAR from the smarter one between the two of them.
It did more than dredge up old, childish fears about things beyond his understanding. It hurt his pride as a provider. As a man. And over the course of a few short years, and helped by a series of long, frantic phone calls back to his family in Ireland, he was convinced.
And that's my personal dream of how the Flynn's biological father lost faith and fled back from whence he came.