Well, if you say he's flexible and leads a "Christian," i.e. morally good, lifestyle, simply talk to him about it, he'll probably understand.
Don't push him to convert, that could drive him off, just express how you feel about it.
If he's as good and sweet a person as you say he is, he'll be perfectly fine with raising children to be Catholic.
I, for one, am pagan, but if my boyfriend decided to be Catholic and raise Catholic kids, I'd certainly help him, regardless of my beliefs.