First, read through the section I’ve already written, on Housebreaking in a Hurry, so that you’ve got all the other elements working in your favor.
Second, make sure you ritually take him out of the house to the same door.
Third, if you’ve followed step one and two, he’ll be anxious about eliminating in the house, and try to “drive” a behavior, which means he’ll start going over to the door where you usually take him out. But this can take a couple of weeks.
You can tie a bell on a string to the door, and after a few more weeks, you’ll see the dog start to hit the bell with his muzzle, or at least bump it, to make noise.But this doesn’t negate the fact that you must always keep one eye on him, until he’s 100%. Another thing you can do is to teach him to bark on command. Then, sit him next to the door, tell him to “bark” and then open the door. Repeat this several times, and you should be able to link the two behaviors.
Pretty soon, you’ll see the dark start to bark in order to get you to come and open the door.