You could train him to ring a bell on your door. Hang some bells on your door and every time you go out to take him for his walk, ring the bell before you open it. Then say some kind of keyword, (potty is the one I use). After a week or so of doing this, start by gently lifting his paw to the bell, or just bump his nose to it so it makes noise. Do this a few days, with the keyword. You will see progress, and reward, reward, reward, everytime he goes to the door or touches the bell on his own. I love the bells!! but beware, at first the puppy will probably start ringing them just to get you to open the door
My AprilLove just LOVED to wait until i would sit down, then she would run to the bell and ring it and watch me jump up and come open the door, then she would sit there and look at me like "haha,, gotcha!!!" LOL, but she did learn eventually to use it for telling me she had to go do her business.