My husband is affected by my flashbacks, too. He cannot be there for me fully because he is reminded of his childhood, where he had to be there for his mother unconditionally. As a result, he tends to lose his sense of self when he's too close to me while I have a flashback. So I have to deal with those intense feelings more on my own then you would think when you are in a marriage. But I find that since I acknowledged that he can't react otherwise, he finds ways and steps closer to me, he experiments what he can do for me without losing himself. But that needs time.
I don't know what feelings you husband is dealing with, but maybe he can't choose to react differently, maybe he, too, is stuck in some pattern, that was established when he was a child.
I think that when your husband tries to not show that he is shutting down shows that he wants to make the situation better, which is good. Maybe you could talk about how to be together when you have those feelings and find another way and be creative about that.
My husbands and my solution is that I go to my room for grieving (a strong trigger for him) and only come to him when I have a concrete question, e.g. please take me into your arms, please listen to me, please make me tea etc.
That is sad maybe, but it is not his fault, nor mine.
So what I had to learn over the years is to take care of myself and nurture myself in flashbacks. I found this list from Pete Walker hugely helpful in this:
http://www.pete-walker.com/13StepsManageFlashbacks.htm
