Not sure if this was mentioned, but I've re-watched Jon's birth scene more times than one human should, and if my skills at lip-reading are as good as I think they are, it sounds and looks like his name is Aerys, which would make sense as Rhaegar's father is Aerys(The mad one). Someone want to take a look and let me know what they think? I also definitely heard that hiss your mouth makes after you say the letter S.
If I can add, Ned renamed him Jon, because of Jon Arryn. Lyanna wouldn't name him Jon. But why change his name? If his name was Aerys, might be a giveaway?