This is a very good question. My wife and I would LOVE to find another COUPLE who think like you do.
Anyway, as stated above, go very slowly. He is uncertain, and possibly flighty. I would explain to B that you are willing to have the whole ceremony and everything with him, but that you can not change the laws and that they are what they are for the time being.
Then, Do a little research into what, exactly, a marriage certificate IS. It is a legal document. Plain and simple. Everything else asside, it is a LEGAL DOCUMENT. So, then I ask, would you be willing to sign a legal document with B? If not, then you are fooling yourself. If you ARE, then explain that to him and go see a lawyer about doing just that. This may help to ease the "equality" struggle B is having with himself. Hope I helped. Take care and much love to all three of you.