Haha, I understand your frustration! My little guy is the same way! I have him trained on a pee pad for inside so he won't potty outside
Two days ago I took him outside to the park where I let him off his leash for about half an hour, and to my amazement, he actually peed on a tree, twice!!
I think that he only did this because where we usually walk, other dogs have peed on the trees...on this tree, no dog had ever peed on it so it was as if it was his very own, haha!
One other tip...the only other time he has ever peed outside is when I took him out with another dog and he watched her pee, then he peed 5 seconds later.
(I originally posted this question about 2 months ago, but didn't really get advice that worked for me...someone suggested taking potty pads outside and praise him when he goes on that but that didn't help my little guy..but might work for you!)