Sounds like there may be some underlying issues that he hasn't addressed yet. As far as the "nothing" and "I'm fine", my husband does this and he truly doesn't think there is a problem. He will be grumpy, snippy, grouchy and non-communicative and will still say everything is fine. To him at the moment it is, he doesn't even realize how he is presenting himself to everyone else. There will be any number of causes from migraines to lack of sleep or stress and each time it's "everything's fine". It takes 20 questions to get him to admit that he has a migraine. UUUGH!!!!!!
It could very well be that he hasn't thought enough about why he is acting a certain way, or even recognized that what he is doing. Tell him what he is doing (be specific and stick to the facts, no sppeculation) and suggest he consider what might be causing it, then tell him you will discuss it later then walk away and come back a few hours later to talk. if nothing else, it will get him thinking.