[QuickSilver]

How wonderful that you rescued him.

When you say that you have tried everything - have you ever watched him so closely that you catch him before or during every accident?

Here is what I did with Thor, with the caveat that he's only had two weeks accident-free so far: I put him in his crate and did not let him out until he pottied. I am pad training (he also goes outside). I had a brutal few days where he would spend 24+ hours in the crate, and I would take him out every waking hour and put him on the pad. I said "Potty" and did not let him off the pad. If he sat or laid down, or didn't do anything after a few minutes, I put him back. When he started pottying, I gave him a really good treat, and let him roam around one room for up to two hours. If he went two hours without another potty, I put him back in the crate and started again. After a few days he caught on and now goes every chance he gets.

I think you could adjust this accordingly if you want to use potty bells, or simply take him outside on a regular schedule.

My working theory is that there is a big difference in these dogs' minds between having just a few accidents and having zero accidents. You need to crate a zone where he cannot fail.