Do you have an X-pen that you could put in your yard?
This has been my experiance when I training service dogs.
First. Depending on the age, they seem to recycle, kinda of like people. You, know a skin baby that was potty trained and then suddenly they start having accidents.
2nd is he in the "teenage" part of his growth, they can be very challenging during this time, down right diobediant(sp).
3rd as they grow things change. They can, and let me emphize
CAN not ALL go through a stage of sensitivity. They can become sensitive to the surface that yesterday they were fine with. Same thing with smells, sounds, ect. It is usually a phase.
Also Keep in mind that usually around 5 -7 months they can go through a fear stage.
What you might try if you don't have an x-pen to put in your yard, is maybe going back to having puppy pads down in the house. Also use praise. When he goes outside where he is suppose to go, make it be the biggest deal ever.
Your neighbors will probably think you are nuts. But it will also keep the neighbors entertained. LOL
Same thing if he goes on the pee pad. You can also use treats. I was taught to ignore the accidents. They reason was that you can really mess them up when it came to relieving, and as we have seen here on YT Yorkies can be a challenge when it comes to this. I have also heard that doxies are just as bad if not worse.
I hope this helps.
BTW I have been putting the link to the tread that you did for new people, on each new persons thread that I welcome to YT. That was a great thread that you started.