Perfectly legal.
There's no telling what stories the lefty protesters are going to make up in order to have the guy arrested.
You could have the whole group saying "yeah, I heard a gun go off." and then on or two people making up a story say "He pointed it right at us!"
Nope, firing in the air in the presence of people is a sure fire way to see the inside of a Texas jail, no matter who's property you're on. Unless this guy can guarantee he knows where the bullets were going to land, he's toast.