Have you tried using the poochie bells? I know some people have great success with them. Jaxon is slowly getting the hang of it.
Also routine is the key. If you can take him out around the same time, he should eventually get the idea. I take Jaxon out first thing in the morning - guaranteed poop. Same thing around 4 or 5 in the afternoon.
It takes time and patience - hang in there!
