Not sure if this will work, but how about taking him nearly indoors, then take him out again smartish so that in his mindset he is going in to do a poo, but does it outside instead!! take a treat in your pocket so that you can give him a treat for doing it outside.
Katie pup is now six months and is nearly reliably clean, she used to do this, out for a while, then in to poo, so i watched her like a hawk and as soon as she crouched I gently put my hand under her bum, picked her up and took her out, and she could not help but 'go' You will have to be careful tho not to scare her in case she then feels she is doing wrong doing it on the pad.
It can be a very frustrating time for all puppy owners - but they do get there, some sooner than others