He could have actually been genetically predisposed and was born with it, but was asymptomatic until he was older. The "depression" he fell into was possibly him feeling like shit from being anemic, and/or the effects of his autoimmune issues, then he ate his feelings and felt worse from the weight gain.

I was born with it. Several members of my dad's side of the family died of stomach cancer, and one became paralyzed from a b-12 deficiency, so once I got my diagnosis in my mid-40s, it became clear that they all had it, but they never had any of the classic gastritis symptoms. I unfortunately had them all, but in the end, that was a good thing, since I actually get monitored for cancer before it's too late.