[Richard]

Let’s talk about the basics, and then you can adapt to your setup. First there needs to be a check valve downstream of the unloaded, so that the system air pressure does not bleed back through the unloaded. Second, any Normally Open pneumatic valve with a 120 coil will work. Wire the valve and pump in parallel so that when the well pump switch comes on, it energizes and closes the solenoid valve. 

That part is easy, the next part of figuring out how to plumb that into your system is the challenge. Some have autodrain water separators, some do not. 

You can also use a standard air compressor unloaded/pressure switch. It too will take some plumbing thought.