I think it's perfectly understandable for you to be hurt when he's first told you about it, and for him to be upset by your reaction, but the thing I'd say is encouraging about this is that he is aware of changes in his mood, is aware that the numbness isn't normal when he's ill, and is aware that his depression won't manifest itself in exactly the same way every single time he experiences it.
As someone who has been there myself, and experienced Depression off and on my entire adult life, this actually sounds to me like someone who does actually have a pretty good insight into his own illness and who is also capable of discerning what's normal for him, and what is not.
There are plenty of profoundly depressed people around who genuinely have no inkling they are ill, so they find it impossible to explain or verbalise their feelings and emotions because there is no context to them. Once these people are diagnosed, it frequently hits them like a ton of bricks and causes all sorts of emotional turmoil, but similarly, it can also be the route to beginning to understand your own head, and can help you start recognise the onset of a depressive episode and take appropriate steps.
I no longer suffer depressive episodes in the same way I used to, simply because I now recognise them at the onset, and I am armed with the knowledge I need to be able to battle it right at the start and overcome it before it takes hold of me. I simply couldn't do this when I was younger because I had absolutely no inkling I was ill to begin with, and just thought that I was extra-moody and temperamental compared to most people, and that my moods had a more profound effect on me than they way most other people experienced theirs. It was only after I was able to put a name to what I was experiencing that I was able to start contextualising it, and start taking some steps to address it.
Your partner honestly sounds to me like he's well down the road of understanding his own head, even though he's possibly still at a point in his life where he finds his depressive spells completely debilitating, but at least he's showing the signs of someone who is well aware of what is actually going on. I'm not stupid enough to think that I will never have to deal with the horror of the profound depressions I used to suffer ever again, but I'm confident that I'm far better armed to cope with it should it ever arise again. Your partner is at a different stage to me, but the things you are saying about him do remind me of a point in my own life where I was still gathering more and more insight into my own illness as I was living with it.