There’s not much that he can do, short of making the environment less hospitable (which is sounds like he’s done by removing the grass). You’ll find various scent based repellents, ultrasonic noise producers, and other claims for repelling snakes, but it’s my understanding that they don’t work. Why not be adaptable? If he’s phobic, there’s not much he can do about that, but garter snakes are completely innocuous, help control mice populations, and they’re pretty cool. I would love to have (non venomous) snakes in my yard. Except for the occasional worm snake, I don’t have any resident snakes.