When should I tell him?

I have been seeing a really nice guy for a little bit now. My past relationship was extremely toxic and mentally/emotionally abusive. I have this urge to be vulnerable with the man I am seeing and let him know some of the stuff I have been through, just so he can have a better picture of why I am the way I am.

How would you go about that? I don't want to scare him away, but I just feel like I want to let him in on that part of me.