The Magnesium Citrate worked, in about an hour. The severe constipation causes prostate inflamation and John had difficulty urinating afterwards, but I gave him Naproxen and within a few hours that problem had cleared up. So we succeeded in avoiding a trip to an emergency room, thanks most of all to what I have learned from online support groups. None of John's doctors have given him useful guidance when he has brought up constipation as an issue. John did say that this time the problem did build up over several days--he thought he was ok after we got back from Germany and didn't pay attention.
My youngest sister arrives with three elementary-school children from California to share the house with us today. When I made our plans to stay on Cape Cod until Friday I thought she was coming Thursday. The house is plenty big but we have our own routine and eat a quite different diet (John and I eat low carb to control diabetes) so I don't look forward to having to cooperate on meals. We will see how my sister responds to John. I've talked more to my middle sister, because I changed my durable power of attorney to list her rather than John.