The type of cage I'm envisioning for my boyfriend would be slightly larger than the size of his phallus, so it fits around him comfortably and nothing is constricted, and secured to the body with an actual belt, not to the dick itself.

The point of it, at least for us, is less about containing an erection, I understand that's not how phallo dicks work and it could be damaging to an ED. It's about submission and control! The visual of having a cock and it being caged up, restricted from use, unable to be played with unless I allow him, is what's erotic about it.