Does he have to have a doggy door? Couldn't he just be taken out the front door on leash to pee, with a ramp if necessary? I don't have my girl go down the steep back steps yet (we may work on that), and only up them once in a while; we always go out the front and down our few front steps for every potty call.
If there's an issue with his "holding it" while you're out without the freedom of the doggy door, perhaps he could be crated when you're gone. He might not love going back to that, the transition might take a bit of work, but considering the alternatives....
Prairieprof, fellow bungalow dweller