It seems to me that the slut or not slut thing is secondary here...you need to figure out if you're ready to get into a "real" relationship with this person or not. Consider that you just broke up, so beware of rebounds. Once you figure out what you want of this person, since you have already ruled out NOT wanting the Friends with Benefits, discuss it with him and see what his thoughts are.
From what you posted, you like him a lot, I can understand why you are concerned about what it is he really wants from you. If I were in your place I woudn't want to just be somebody's booty call when I have feelings that are stronger than the usual physical attraction.