Thank you for the additional info. He sounds like he's been through an awful lot with his past relationships. By the same token, you have been with him now for a long time now, and there should be some element of trust that has built up. Hopefully he is somehow seeing that you are not like that, and that over this amount of time you have shown him that you are on his side and not willing to hurt him in any way. The only way to really move forward from this is to talk to him consistently and let him know that you've been hurt, we've all been hurt, but at some point we have to learn to trust again.
Additionally, you are not the one who hurt him, so it's unfair to punish you for the sins of other people. He made the decision to be with those people, and yes they hurt him...but again, it wasn't you. It may be good for him to seek therapy or counseling again, to see if he can overcome his fear of being hurt, and learn ways to deal with you more openly.
It might be a good idea, if he's open to it, to sit down and write what about his hope and fears for the relationship. What does he hope will come out of the relationship? What are his fears about the relationship? You can also ask him questions, such as, do you think I would hurt you? Have I shown you that I won't hurt you? What would it take for you to trust me? Do you have a time frame on trust? Do you think I have been hurt? Do you think I should hold you to the standards of other men? Or blame you for things they did wrong? And see what his responses are.
You can only be you, and he can only be him, but after a certain point, you'll have to decide how long you are willing to wait, how long you are willing to be kept at arms length while he decides whether or not to trust you, and how long you will have to pay for others mistakes.
I welcome your thoughts, let me know if you want to talk more