First find the times he normally poos at e.g. first thing in the morning, half hour after meals, early in the evening, et cetera. Once you know these times take him out on a leash and go to the spot you want him to poo during the times he normally poos and wait two minutes. If he doesn't go within that time take him inside and put him in his crate (or other such place you know he won't poo) and try again in half an hour. While you're waiting to take him out again don't play or interact with him. If when you take him out again and he still doesn't poo within the two minutes take him back inside and try again in another half hour. Keep doing this until he does poo and within a few days he'll learn that he has two minutes to do his business and if he doesn't he'll have to wait till next time, no games allowed.
Sometimes, the smallest things bring the greatest joy.
I agree with everything LB said plus I like to add a command. I use "go potty" because it isn't something I say in normal conversation. I take my dog outside and give the command. When they go I'll say "good boy/girl" in a very calm and quiet voice. Also, for me, walks are not potty breaks. If I am taking my dogs on a walk or to play at the park I let them out in the yard to do their business before we go. It keeps me from having to clean up after them (I still take clean-up bags just in case) and less worry for me that they will need to go in a less then desirable spot.
I give mine a little more time than 2 minutes, probably at least 5. I also use "go potty" for their cue. My dogs have never pooped while out walking on leash.
Seldom will they even go if we are walking in the field for any length of time. But as soon as we get in the fenced area by the house they all have to go.
Be patient and put a word with it. Always use a leash and go to the same area. Consistency is key.
One Border Collie Is Never Enough