If you activate an android phone first (needed anyway to earn silver since there is no flipphone app) and then email firstname.lastname@example.org they might do it. No one has tried as far as I am aware. They let iphones on that way but an iphone app is coming soon. You'll have to ask them if it is possible. There is no way you can directly sign up using a flip phone since the app is required for initial activation.
Maybe cellnuvo support could chime in if around please.
If he doesn't use much then tello is probably better for a flip phone. Or TPO if you want unlimited talk/text for $7.99+ tax.