How should I let my maybe-boyfriend know I’m maybe poly?

The same way you let anyone know anything. You’ve got to take a deep breath and just say it. Be honest and clear. Tell him you like him and want to see where things can go between the two of you, and you’re also thinking about exploring polyamory. Ask him how he feels about all this. Be prepared to listen and learn about him, and to answer his questions openly. Sometimes the most exciting things in life grow out of “maybe”s. Good luck!