Amy,
There are many ways to go about the DNR issue. One is to get a form from your doctor (doesn't matter which one) and fill it out, have him sign it, you and John both sign it, and then make a copy for his hospital records, and frame the original, and hang it where anyone coming in the door (like visiting nurses or EMT personnel) can see it. That way, if for some reason you are out and someone needs to know about this issue, it will be covered.
The other thing is, there are varying degrees of DNR, one of which includes a DNI component. For instance, if the problem is just a breathing issue, would you want John to be intubated? If it is a cardiac issue, would you want CPR to be done?
What about feeding and IV hydration? These are questions you will be asked, and it is better to have "thunk" (I never get that right anyway, thinked, thought....) them over beforehand when the question hits you.
The kindest thing my parents ever did for my sister and I was to get these issues dealt with while we were all still able to talk them through.
Having said all this, I sincerely hope that it will be a long time before you actually have to use any of these pieces of paper, but it is good to have the writing of them behind you.

Andrea