Susie, first off let me say that (to me) this sounds like a familiar pattern. I was a victim of sexual abuse as a child, and every relationship I've had has been characterized by an uncomfortable ebb and flow, an opening and closing of my heart and my capacity to express and even feel love. It's such an unfortunate pattern and no doubt very difficult for you to hold on to.
Being that as victims, we were forced to provide affection, to please and devote ourselves to the wants and needs of an adult while totally disregarding our own feelings, has resulted in an inability to devote ourselves in a trusting manner to those adult partners in our lives that truly deserve our love and are not trying to force it away from us... It's confusing. For me it becomes particularly difficult when my partner is asking for attention, as the hurt child in my heart turns deeper into resentment and refuses to "comply" because for once he has that power and ability to say "No!" Same goes for therapy, he is holding on to it and protective of it in a way that may make him perceive your request for him to share to be a violation of his trust.
I know this is hard. And I know that you want to understand him and support him. And it is brave of you to take this on. But you are also in this partnership and also need to be taken care of.
I would recommend that when you have an opportunity to have a discussion with him, that you share your experiences. Do so in a way the emphasizes those times when he is loving and caring, focusing on all the warmth and affection he is so capable of providing. You don't want to make him feel bad for not giving, but rather point out how much he does give, what it feels like to you when he is able to give, and then share the experience of having that go away. In that context, try to find ways together in which he can remind himself, and remind you that you are loved even when he feels completely shut down. For me, for example, I've found that when I am shut down, having my partner rub my back until I feel ready and safe to come out again seems to work, and then my heart is able to open and I turn and rub her back. It's kinda silly ceremony, but it's powerful for me. It makes me feel like I am being unconditionally loved in a non-threatening way, so I don't have to stay protected. Maybe you guys need to talk and see if there is anything that eases the heaviness of his heart even in the slightest, and focus on that to find each other again in the midst of his most fortified moments.
Remind him how much you work to understand him (like spending time on this message board,) explain to him that you do this work out of love for him, and ask that somehow in his process that he try to do the same.