I’m a little surprised at how harsh (at least to me) some people are being. Depression itself is not a sin or bad habit, and I didn’t see any indication he is exhibiting nondecent or irresponsible behavior that people are making excuses for. Nor is it a ‘death kiss’. There are many people who are able to live healthy lives and have healthy relationships despite suffering from depression. Nobody has all their pieces in the right place.
What can be a problem is how he reacts to it, and how much effort he puts into controlling it. I’m definitely not advocating going into the marriage if he isn’t even trying. But from OP’s words he IS making strides towards that. So while they may need to postpone the marriage to make sure he can function well, take care of children, give himself to his wife fully, etc - I wouldn’t say this makes him not called to marriage.
Also, just because we haven’t heard of something doesn’t mean it doesn’t exist, especially in the realm of science and ESPECIALLY in the realm of the brain and mental health. I think getting a concussion, regardless of where it happened, cound be traumatic in an of itself.
If he was using it as an excuse to sit around and mope, or to be mean, or lazy, or blaming all of his problems on it, etc - then yes, that is a problem. But again, it seems like he is making positive strides. It’s not going to be easy and sometimes he’ll fall back into unhealthy thinking, but that’s when it is most important to show him love and support and help him be the man he is called to be.
Sorry if I am now getting upset on somebody else’s behalf but it hurts to know that I would be so dismissed and judged as unfit for marriage if you knew me a few years ago

(In all fairness, I probably was, and I knew it, and I worked at it. And I’m glad my then best friend and now fiancee didn’t just give up on me and consider me a ‘death kiss’)