I’ve seen them. They are annoying for customers, too.
I would consider a few different options.
1. See if he can get a reporter on the local paper to write a negative article about it.
2. Talk to his local representative about a noise ordinance. (If he refers to it as noise pollution or if he can show that it affects some sort of wild animal the environmentalists will swarm in and take care of the problem for him. Maybe it interferes with the mating calls of birds?)
3. March outside the station with a sign encouraging people to go elsewhere for gas. If he can affect the owner’s profit, that might make the owner think twice.
He could purchase noise cancelling speakers, he could wear ear plugs, he could sound proof his fence, walls, windows etc...