How do i get him to be just more than friends with benefits with me?

I've been "fwb" with *dan* for only a couple of times. i've known him for about 4 years. i think we've always had something for each other. but one of us was involved with someone else. this aug we started hooking up. ive mentioned that i wanted some kind of commitment but i don't think he really took it seriously. hes a very closed person and doesn't let people in very easly. is there something or anything to let him know that i really want to be with him and that he can trust me?