Hate to break it to you, but:
If Serebii says something like that, it's usually correct. I checked other sources, and they're right - Flygon can't learn Aerial Ace in a Generation 3 game. (It can in Gen 4 onward, however.)
....All does notes I have been taking about movesets for my pokemon like Flygon... *sniff*
Okay I don't have a plan B..but I might have to breed it so it can learn Quick Attack or think of another role it could play.
I don't have a Flygon but I do have a tranpinch (in training)
You could always teach it fly as an alternative. It may seem like a waste of a turn, but that is only if you are impatient. You could use that turn as a Leftovers heal.