Been a while since I had to "poddy train" a pup -
my "Rescues" have been mostly pre-trained.

A suggestion >

Next time you think he's "ready" to *go* -
Leash him up, grab a Good Book and a Comfortable Chair...
And set up camp near the preferred spot!

And then just *wait him out*!

You might get several chapters read;
but sooner or later, he's *GOT to GO*!

/s/ Phred