it sounds like we had similar situations!! I have no clue how, but when my pup came home with us from the shelter at five months he seemed to know already that the inside was not the place to relieve oneself. That's a HUGE blessing, and congrats on having a wonderful pup!
One method I used myself before we changed houses is super easy to train and even the most... *ahem*... 'goofy' sort of dog can grasp it. (aka i've trained a beagle to use this method in the past)
I'm not sure where you could find them in a store outside of christmas time, but I'd root around for one of those doorknob-hanging belts with sleigh bells on them. Any sort of bell works, but the sleigh bells on the strap keeps the door from getting scratched/damaged and they're very easy to get noise out of for your dog. Be sure they hang down low enough on the door to reach the dog's nose without encouraging him to jump up.
anyway, put the bells on the door you'll be using to let him out most of the time. Every single time you let him out to do his business ring the bells right before you open the door. After a while you'll find that if you ring the bells while he's not there he'll come running. Most dogs will naturally progress to figuring out "hey, if the bells ring i get to go potty!!" and learn what they have to do to make 'em ring. If not, encourage the dog to accidentally ring them until he realizes that he can do it himself.
during the early training stages be sure that every time he rings the bells of his own accord you open the door immediately, let him out, and praise, praise, praise! As he gets older and you're more confident in his potty-training status you can ignore more of the bells, as dogs then see that ringing the bell brings their human AND can get them outside to chase squirrels! Intermediate reward is a stronger motivator, anyway.
there's only two downsides to the bell training. Your door can get quite scratched up, especially if an excited dog likes to ring with his paws instead of his nose. and if the dog is on the intelligent side you could start hearing bells a LOT more often than you'd like, especially at three in the morning. When we moved to a place with a smaller yard (unfortunate, I know) we had more wildlife visible to him from the house. If there was a possum or chipmonk or anything small, furry and lumbering we'd hear the bell at any ungodly hour. It became more of his 'i want to chase things, lemmie out' bell. since he's getting to be on the old side we took it down and just take him out oldschool four times a day.

hope this gives you some ideas... and congratulations on the new pup!