I vote that there is no good reason he HAS to be told- why tell him something that will make him sad and angry? Does it really matter that he be told this upsetting information? Which he will most soon definitely forget? I use "the doctor wants you to stay here for now to get the extra support" (his PCP's suggestion). Then you turn the conversation to fun memories of things we did at the house when ...We've laughed and laughed about the time one of the kids.....or when Mother dropped the lasagna....
Who is telling him helping? Telling him will make YOU feel better as you tell yourself you're being honest, not hiding information, etc, but this is the time to think about him and his feelings.
I used to work in a SNF where a devout Catholic lady with Alzheimers would become quickly agitated that she needed to catch the bus to meet the Pope (he did come to Boston). We would say that he was running late and would be there to meet her. She would calm immediately. Did she need to be told the truth? It's called "Validation".