I have cared for my mother-in-law for more than 5 years now. She too is a DNR and she has had strokes, heart problems and last year the day after Thanksgiving, she broke her hip. I fully understand your delimia. She, too is a DNR. We've now had multiple converstaions about
what she wants and doesn't want regarding 'critical' care.
If your Dad is a DNR, does he also have a POST or MOST(physicians's/medical orders for scope of treatment) or a living will/advance directives? If so, do they indicate what he wants done/not done? Such documents allow a person to specify if the WANT IVs, artificial nutrition, etc. as well as dealing with DNR issues.
If your/his insurance has a 24 hour nurse line, that would be an excellent place to start.
Contacting his Primary Care Physician is an excellent idea, as is contacting hospice. There are many things that can cause sudden, dibilitating weakness, espeically in a fragile elder. You were right to suspect dehydration and congestive heart failure, but if he is not showing other symptoms of those, there are many other things a physician could check. Does he have a history of strokes or heart or kidney problems? What has his appitite been? Is he keeping food down?
If you have a wrist blood pressure cuff, I'd check his bp and pulse with that. And check his temperature. The more info you can give his doc the better. Also- hold his hands in yours and ask him to squeeze both at the same time- see if they are about equal in strength.
With his symptoms, you'll be sitting in an ER waiting. He's not bleeding, having trouble breathing, or showing obvious signs of heart problems or a stroke, so other cases would probably be seen first.
Prayers going up for you both now.