I lean towards thinking he's fake but I'm not totally sure, and I don't think it's as clear cut yet. The reasons can be explained away, to me at least, enough to make it possible that he's the real deal.
Firstly, the idea that the baby swap only makes sense in retrospect. I don't think so. If Varys wanted to save the kids life, it made sense to take him, there didn't have to be a swap but that would make getting him out and keeping him alive easier (and is well within Varys' capabilities to pull it off imo). The big suggestion is people would have known if Gregor hadn't smashed the baby's head - maybe, maybe not. Do we know for sure that the infant had blazing purple eyes and silver hair? These things often change in childhood.Infants look fairly alike, and most of these people, Robert, Ned, and so on have probably never clapped eyes on Aegon in their lives. Jaime, I suppose, maybe the only one who would know. Clegane was sent to kill the baby and however he did it, a baby in Aegon's room, in Aegon's cot is going to be evidence enough that it's Aegon, I think.
Secondly, the Aegon Blackfyre theory. I don't really remember all the details of all the history and family trees, people seem to think Illyrio could have had a kid with a Blackfyre woman so I'll take it from that their line exists in some form (anyone who can clarify this for me, please feel free
). Wouldn't it be something of a phyrric victory for people loyal to Blackfyre to put a their King on the throne with the whole of Westeros, including the kid himself, thinking that he's Aegon Targaryen? His dynasty will always be called Targaryen, for all intents and purposes that's what they will be. Even if the Blackfyre's would have taken the Targ name had they won, that is because they'd claim to be the rightful heirs. But no one will ever know about this victory, if the Golden Company is remembered it will be for putting a true Targ on the throne, not a Blackfyre. So what do they win? Glory, lands, and wealth under a new king, their king, The same prize whoever he is, Blackfyre, Targryen, or long lost Lannister! Even if it were a Blackfyre 99.99% of them wouldn't know it. Only the leaders would know, maybe not even them. So following a Targ, not that big a problem for the Golden Company.
Thirdly, Varys and Illyrio, why help Viserys and Dany at all? Even if they thought they weren't helping them, sending them off with the Dothraki, why not just sell them out to Robert or get rid of them some other, quicker, easier, deadlier way? Unless they are keeping them as a backup option - but that brings me back to my previous point re: the Golden Company, would they do this is the Blackfyre point was so important to them? The idea of Varys being a Blackfyre and therefore fueling Aerys' madness... perhaps. But who accuses him? There's a lot of people dislike and distrust Varys, people that were maybe loyal to Aerys, people looking for a scapegoat and finding an easy one in the spider. This version of events isn't neccesarily the asbsolute truth. Even if it is, Varys' motives are still not entirely clear. Even if the Blackfyre theories are right some of his words and actions still don't quite make sense. And why lie to Keven Lannister, a dead man?
"Aegon? Dead. He's dead."
"No. He is here."
Finally, my weakest points (I feel
). The reasons for Jon Cinnington wanting to believe Aegon is real are crashingly obvious, his eye colour and hair colour can be explained away too. But Griff did know Rheagar and Elia well, he knew their faces. Would hair and eye colour be enough to fool him, or does Aegon resemble his possible parents even more closely than that? If he does, that casts doubt on at least two or three theories of his parentage!
But then there's the prophecies and visions. Enough of these have been right to show that they exist beyond doubt in the world. Some I think will be proved wrong before the game is over, and they can clearly be misread, or just so ambiguous that people (readers and characters) can apply them to whoever they please. There could be other liars and false dragons. They could apply to Ben Plumm!
But that doesn't seem too likely.