No way of knowing. There are a couple of theories. Warning: Spoiler! (Click to show)
It's also unlikely that JonCon himself knows. He was not present for the boy's entire life. JonCon fought with the Golden Company when he began in exile and only later learned of the boy. He loved Rhaegar so much that he'd believe anything.
There is a belief that Aegon is the mummer's dragon that Dany sees in the House of the Undying. Varys was a mummer, so if Aegon is his plot - along with Illyrio, then he is the mummer's dragon.
"A faceless horror of bone and brain and gore. None of us looked long. Tywin said that it was Prince Aegon and we took him at his word." Kevan Lannister
Warning: Spoiler! (Click to show)
I probably have to read these again.