Sounds like you guys really care about each other. And it sounds like he may be using this FWB "joke" as a way to enter a relationship. If you are not comfortable with being in a relationship, I would stay away. I have tried FWB before when I was being completely serious. They thought it meant otherwise (even though I spelled it out that I didn't want a relationship) and eventually I had to break it off because they wanted more. And then it would also ruin the friendship because they would inevitably be hurt.

So I will throw that warning out there. It's very hard (especially for women) to have sex without it turning emotional. To me it sounds like, to him, this would not be strictly for sex. And it sounds like it really wouldn't be strictly for sex for you either.

I would have a very blunt conversation with him and get everything out there. Hopefully he will be completely honest (as I hope you are too) and things can get figured out. It sounds like you guys would be good together if you can tell him anything already?