How do you trust someone with a past?

We all have pasts...but as deceitful as his...He has never done anything to me to break my trust but....ugh Let me start from the beginning.

I have this guy friend....lets call him Dade.
Well you see Dade and I have been friends for a long time...and he has told me a lot...Like the fact that he has cheated on every girlfriend he has ever had, and has told girls that he loved them when he didn't. Now we have had a thing for a while...but we are not in a relationship, we have gone out(a few weeks ago) on a "date" once before....(yes we have hung out alot since then)

He has asked me to be his girlfriend a while back like in October...but I turned him down cause I had a friend that had feelings for him...but I have not spoken with her in along time now...since I droped out of school in Nov...I don't talk to any of my old school friends cause I really don't want to.
Now there is nothing holding me back (other then the fact that he hasnt ask me to be his girl since heh)
But what if he does? What if I say yes? What if he cheats and lies? What if I let him in to my closed heart? What if I let him in and he brakes it? But what if he wouldn't? What if I say no? What if I miss the chance to be with him? What if he really would have been faithful?
The thing is I really can't see him being a cheater...at lest not anymore. He is such a great guy...I have spoken with on of his friend...cause he was nice and gave me a ride home and he was saying how he is one guy you can really trust! That he is the type of guy that has your back not matter what? (No, I didn't tell his friends how I feel about Dade)
My god...I really do think I could let him in....but if I do? I'm really kind of scared of what he could do.
I have never been one to judge someone or hold them to what they have done...only for what they do. But what if I do let him in...and he hurts me?