That's how my bf thinks. I think it's just being very open about religion. Problem is I dont think I am (though trying to be if that makes sense) and I don't think I'd feel that his conversion is genuine if he still believes in everything else.
I feel you. I actually would want my bf to convert to Catholicism (even from Protestantism) for cultural reasons even though I say all religions/denominations are valid and I'm not the best Catholic in the world. It's not a deal breaker for me, but it's important.

Who knows? Maybe your bf will actually be genuine in his conversion. Maybe he'll study it and be like "Wow! This is the truth!" Sometimes converts are the best Muslims (Christians, Buddhists, Wiccans, etc).