Just apply! Availability is gonna depend on your local, I know mine has doubled its class size over the last few years. We're in high demand and young folks have realized it. But if he can show up, pay attention, and piss clean im sure he'll do fine.

Anything he can learn about basic electrical theory will definitely help, but its not strictly necessary. They'll teach him everything he needs to know. Still, being familiar with stuff like Ohm's Law wont hurt with the interview.

I believe the classes at my local start in September so it may take a while, but if he's patient its worth it. The union benefits are amazing, he'll be set for life and able to work pretty much anywhere he wants.

Meanwhile, if he can find a job as a helper he'll get valuable experience. Won't be a well-paying job, he'll be doing a lot of grunt work (up to and including literal ditch-digging) but itll give him a view of life outside the union and appreciate the IBEW more with any luck. He'll need some basic hand tools to start with (11-in-1 screwdriver, impact driver, lineman pliers, wire strippers, diagonal cutters, sharpie/pencil, razor knife and tape measure), but id suggest avoiding buying more power tools until he knows if hell be union or not. If he is, theyll provide them. If not, itll depend on his company. But a helper/apprentice shouldn't worry about anything beyond a drill for a few years.

Hope that helps! Feel free to ask any other questions and ill do my best to answer. Ive been in the trade about a decade and im self-employed currently.