My boyfriend says he has difficulty trusting and feels like his heart is made of stone?

It sounds to me as if he could benefit significantly from therapy. If he is being this hard about opening up and trusting you he definitely has some issues that he could benefit working through. If he isn't willing to open up and trust you, uming you have never done anything to loose his trust, then he definitely needs some professional help. If he is unwilling to trust you, then your relationship is unfortunately doomed because a strong and healthy relationship is based on a solid foundation of trust.