The most important lesson I learned in my nurse's training was to listen and acknowledge. Patients are in an abnormal circumstance and at the mercy of their caretakers. We cannot tell them what to do or what is right, but we can listen to what they are expressing and acknowledge what we think they are trying to tell us. If you can't figure it out, it is OK to say that you want to help, but don't know what is wanted.
Having been on both ends, I can say that just knowing someone cares means more than if they can actually do anything. Try not to lash out when he says stupid things. Best to just walk away and come back later, explaining that you want to help but cannot if he is going to be abusive. Just know it is his frustration that is causing his outbursts, not you.
Of course, shutting down is actually worse than taking it out on you. It means depression that is deep and serious. He does not necessaily need to talk to you about what is bothering him, but he needs to get his head out of the dumps before he is circling thedrain of despair. He needs to know that marriages have been ruined over chronic illness, but it does not have to happen.
Jan