SSA does not usually approve a person for benefits, based on a diagnosis. There are people diagnosed with bipolar disorder who are able to work and earn enough money to live on. Other bipolars can't support themselves. Your husband was not granted benefits because he had such-and-such diagnosis. He was granted benefits because the thing he is diagnosed with is causing signs and symptoms that interfere with him performing adequately on the job.

You mentioned concern for your husband's self-esteem. Getting into the nitty-gritty of how and why your husband was performing poorly in work venues might not be pleasant for him to contemplate. As far as what to tell people - it's not most people's business to know. Your husband "retired early due to health issues." I wouldn't get any more specific than that.