It's best not to mix AQ salt with medication. Before you treat, are you positive it's velvet? Even if you cannot tell by color/flashlight test, it literally looks like dust stuck to the side of the betta. When Rose and Tiersias had it, the velvet was really obvious. The velvet sticks to the betta similar to dust on a table.
I had a fish a two weeks ago with the same exact symptoms of your fish, except she did not move at all. She was also twitching and flashing, so I assumed it was some sort of external parasite. Fortunately, she got better with AQ salt treatment.
How's is his poo? X]