I am so sorry to hear, it really stinks! Do you remember where he came from?
I see this randomly here, especially in the "given up" birds. Some of my 2nd year flock I had loss from, exactly same situation. I hate to say it but I am learning the "common" hatchery birds often don't have longevity. I see it more from certain farm stores, especially RK. around the 3rd year or less, they have prolapses, flips, just up and die for no apparent reason. Without necropsy, you will never really know the cause. I rarely see surprise losses in birds I have that are straight from a known breeder that takes his/her breeds seriously. Its part of breeding responsibly. Only choosing the strongest, liveliest to continue your flocks. I had 15 lavender orpington from a very good breeder. I weeded down to a trio, and kept a 2nd rooster I was equally impressed with. I cull hard in the breeds I plan to keep for many generations! The culls, I give away for pets, layers etc.and only sell the birds/eggs I would be breeding myself, but have no room for!
I have some hatchery birds that are fairly well bred, and most hatcheries do take pride in keeping a good stock. Its the 50 cent chick companies that cost us more heartache and headaches in the long run. Several of us have debated if genetics were worked towards shorter lifespan for a faster turnover! I find the 50 cent bin chicks are the least likely to see adulthood, or much beyond it.