Why does he only want to be FWB when I know he likes me?

If he came right out like that and straight up told you he just wanted to "fool around", there is no doubt that's ALL he is interested in; using you physically is his end game, nothing positive. Don't let it go too far that you'll regret it if he leaves once he's gotten what he wants. You're worth so much more than that! You deserve someone who will appreciate you for your personality and character, not a means to an end (physical enjoyment). Don't ume he really has good intentions, because if he lays it out like that, then that "fooling around" is truly all he wants from you.