Most dogs need to spend a bit of time wandering around and sniffing before pooing, also a brisk walk, 10 mins or so will get things going. So it might be easier to just take dog out a couple of times a day for a short "poo walk". He'll soon get into a routine. My two dogs refuse to poo/pee in the garden, we're lucky to have a local dog toilet area, grass, poo bins etc nearby and they always go there.
I trained one of mine to go in a specific area by taking her on the lead and praising and treating her for going there so it can work.
My younger dog on the other hand, has a real "poo ritual" that involves much sniffing and very careful selection of his poo spot. I must admit I gave up quite quickly on training him to use the "toilet" as it wasn't important enough to me to spend ours out there with him.
Eric has decided in the last week or so that he will do all poos on his walk rather than the garden. This morning he brought me his harness to get me to walk him purely I think because he needed to poo. Could you make walks coincide with the times he needs to poo? Eric poos on first and last walk normally, the early one this morning I suspect was because of dh snacking and treating dh lat night when I was in bed.
But don't come back on here and flame me if it doesn't work. . It's what I would try first. You might have a bit of control if he's on the lead. Once he's done it in the same place a few times he might remember the scent.
Our 8 month pup is doing so well. We moved his puppy training pads outside to a designated spot in the garden and he did all his toileting on there. But now he ignores the pads and poos freely all over the lawn. So we have to go around spotting it and clearing it before using the garden. I'd love it if we could get him to go in a single spot, without the treasure hunt! Any suggestions?